On the Survey of “La Escollera” and a Submerged Structure Related to Fort San Matías in the Bay of Cartagena de Indias

This research analyzed and characterized the structure built in the mid-eighteenth century to close off the Bocagrande in the Bay of Cartagena by using written and cartographic historical records and employing multibeam acoustic technology. This submarine wall closed the navigation through Bocagrande permanently, from the nineteenth century to date. The analysis made it possible to estimate the dimensions of the structure in its current state. It also reveals its relationship with the archaeological context, involving other structures, together, provided defenses for the city and the bay by protecting the navigation through its largest mouth. The structure is accompanied by two depressions in the inner part of the bay—one, a parallel trench along the entire structure, and the other more remote and tenuous—both caused by hydrodynamic factors such as current scour and wave effects. The multibeam images show the ruins of the foundations of a bastioned fort with 82 m sides, at an average depth of 6 m. La Escollera (the breakwater) was later built over part of the structure. The comparison of plans from 1769 to 1786 and the bathymetric survey carried out in 2021 suggests that this structure corresponds to San Matías Fort, built in 1567 and dismantled in 1626, one of the lost military fortifications in the Bay of Cartagena de Indias. The analysis also permitted argument on the location of Santangel platform and of Point Icacos.


Introduction
Cartagena de Indias, Colombia, a port located in the north of the country, offered a strategic advantage to the Spanish crown as it conquered and colonized territories in the American continent.Cartagena was a vital commercial and cultural center in the Caribbean from the sixteenth to the nineteenth centuries.The city's port was a hub for the export of valuable commodities, including coffee, tobacco, and sugar, to European markets.The city was also a significant center for the slave trade, with enslaved Africans arriving in Cartagena and then being sold to plantation owners throughout South America.The port played a crucial role in connecting South America to the Caribbean and the rest of the world, and its influence extended beyond commerce to politics and culture (e.g., Del Castillo 1997;Segovia 2009).
The city of Cartagena de Indias sits on the northwest coast of Colombia at latitude 10°23′N, longitude 75°23′W, on the Caribbean Sea (Fig. 2).The bay of Cartagena has a surface area of 82 km 2 and an average depth of 16 m.The space between the city of Cartagena in Castillogrande and the island of Tierrabomba is called Bocagrande (Fig. 2).Between these two places is the continuous underwater wall known as La Escollera, which was constructed at the end of the eighteenth century as an obstacle that prevented the navigation of large ships through that mouth.Bocagrande has an average depth of approximately 6 m.
Cartagena is an important cultural center, and was so almost since its foundation, as the main Spanish port on the mainland Caribbean of the continent.Its architecture and urban planning were heavily influenced by Spanish colonialism, resulting in a unique blend of colonial and Caribbean styles.The city's cultural scene is also vibrant, with artists, writers, and musicians flocking to the city to engage with its unique mix of Spanish, African, and indigenous cultures.As a result, Cartagena became an important cultural center in the Caribbean since the nineteenth century, with a legacy that can still be felt today.
The strategic importance of Cartagena extended beyond commerce and culture, as the city served as a critical military stronghold during the nineteenth century.Cartagena's fortified walls, built by the Spanish to protect against pirate attacks, proved to be a formidable defense.The city was a crucial bastion for the Spanish crown during its conflicts with other European powers, including the British and the French.Its strategic location and robust defenses made it an essential center of military power in the Caribbean during the nineteenth century, influencing the outcome of many significant conflicts in the region (Kenneth 1987).The fortifications of Cartagena date back to the sixteenth century.Attacks on the city from the sixteenth to the eighteenth centuries forced Spain to develop a plan to defend the Bay.By order of King Felipe III, the military architect Bautista Antonelli oversaw the construction of fortifications in the city (Tennant 2003) based on a plan that began in 1602 with the bastion of Santo Domingo, the Santangel platform, the Santa Catalina Bastion, Fort San Matías, andCastillogrande Fort in 1626, among others (Salas 1992;Redondo-Gómez 2004;Llopis et al. 2020).
Some of these forts disappeared for different reasons, such as destruction during major battles, lack of budget for maintenance, and the influence of the climate in each region (Del Cairo-Hurtado 2014;Quintana-Saavedra et al. 2022).San Matías Fort, which was dismantled in 1626, was one of these.Its role in the city's defenses was undertaken by the Fort of Santacruz de Castillogrande, located further inside the Bay.
The city of Cartagena, overlooking the waters of the Caribbean Sea, from the first day of its foundation, was a strategic point of utmost importance (Arciniegas 1983, p. 46).The defense of the Bay of Cartagena de Indias was a matter of concern for the Spanish crown, given the condition in which the city's defenses were left after naval actions during the War of Jenkins' Ear (1739-1742) which bled into the War of Austrian Succession (1740)(1741)(1742)(1743)(1744)(1745)(1746)(1747)(1748) in Europe (Bushnell 1981, p. 32).The nineteenth century arrived in the Caribbean with the winds of independence for the Spanish colonies, whose first manifestations began in the port of Cartagena de Indias in 1811 (Marchena Fernández 1998, p. 87).Cartagena de Indias was one of the most important locales in the struggle for independence in the Caribbean region (Liévano Aguirre 1989, p. 65), with the consequent reactions of the Spanish crown sending troops to Cartagena de Indias in 1815 to suppress the independence movement, which triggered a series of conflicts in the region.
Fortifying the Bay of Cartagena for its defense as a key strategy for defending the city was vital due to its ample deep spaces with narrow access channels, protection from the winds by elevations of land, separated from the mainland by swamps and long channels with dense vegetation that made it difficult to access, with available resources for construction, and the permanent waves that safeguard the entire coastline (e.g., Arrazola 1961).
The Bay's natural conditions and advantageous strategic position made the city of Cartagena de Indias founded on its shore in 1533.From then on, Cartagena lived under constant siege from the enemies of Spain due to the wealth it administered.For this reason, it was necessary to advance a fortification plan, which great engineers in Europe carried out in stages, making it one of the most secure ports in the Americas (e.g., Segovia 1992).
The Bay of Cartagena de Indias is protected by Tierrabomba Island.It is separated from the land on its north end by a wide, relatively shallow channel called Bocagrande, and to the south by a smaller and deeper channel called Bocachica.Bocagrande, the natural entrance to the Port, was opened and closed several times over the centuries, depending on the currents that altered the littoral drift of the sands flowing south from the Magdalena River delta (e.g., Vernette et al. 1984;Andrade-Amaya et al. 2013).The lack of sand-and the consequent opening of Bocagrande-is often due to natural causes such as obstructions to the flow due to obstacles along the beaches or extreme waves due to a storm, which severely erodes the sand bars to the point of breaking them, as it had happened with time (Andrade et al. 2022).
The first maps show Bocagrande open (Fig. 1a).It is through this mouth that Francis Drake's squadron entered, taking the city in 1587 (Fig. 1b).Over time, the mouth was closed naturally by the formation of a sandbar from the north, in the center of which lay Fort of San Matías, which was constructed in 1627 (Fig. 1c).As time passed, connection to the south was lost.In 1642 the grounding of some ships in the narrow channel led to its closure (Fig. 1d).The maps made at the time by the French Squadron of the Baron de Pointis when they entered the Bay in 1697 show Bocagrande closed (Fig. 1e) and those that show the Bay during the English attack of 1741 under the command of Admiral Edward "Old Grog" Vernon show a small channel in the sandbar of Bocagrande (Fig. 1f).This sandbar gradually vanished from the surface, as observed by the commission of engineers that arrived to repair the city's fortifications after the 1741 siege (Fig. 1g).As a permanent solution to the problem that left the Bay unprotected whenever Bocagrande remained open, the decision was made to build a submarine stone wall structure, from the mainland to the island of Tierrabomba.The project became known as "La Escollera de Bocagrande" (Fig. 1h).
The construction of the Bocagrande Breakwater was order by the Crown on 15 October 1764.The project was entrusted to Antonio de Arebalo (curiously, he spelled his last name with a b, instead of the more common surname Arevalo), a mathematician and military engineer appointed to design, direct and complete the fortifications of the city of Cartagena de Indias starting on 11 November 1771 and ending in 1778.Arebalo's project called for structures of various types.At its foundation were sectors of one, two, or four lines of parallel pilons forming caissons filled with stones buried in the bottom of the sea on either side of the sand bar that still existed, these formed caissons, which were later filled with rocks.On top of it came a first layer of small rocks, then a second layer of larger rocks.The upper part, finally, was crowned with the largest rocks (e.g., Arebalo 1769).
This structure survives to this day: it appears on nautical charts and is visible from the air.The visibility is generally poor due to the water's high level of turbidity.This situation makes it difficult to conduct a detailed evaluation of its conservation status by direct observation.The arrival of multibeam systems to the Colombian Hydrographic Service allowed a better visualization of this structure as shown here.
The exact location of Fort San Matías became blurred over time, since various authors have established its position based on descriptions collected from older documents, many of which said, for example, that the fort was on a site called Punta de Icacos (Segovia 1992, p. 24;Porto 2016, pp. 13, 16, 17).To this day, Point Icacos has always been assumed to correspond to the spot currently occupied by the El Laguito neighborhood of Cartagena, although other sources indicate that Punta Icacos was on the island of Tierrabomba (see Fig. 2).This geographical point has therefore tended to distort the position of the Fort.To confirm the position of the ruins of Fort San Matías, plans from the seventeenth and eighteenth centuries were compared with a bathymetric survey carried out in 2021 using a multibeam echo-sounder.

Study Area
The origins of the shoal water where La Escollera stands (Fig. 2) dates back to the formation of natural coral reefs extending parallel to the coast and consolidated towards the end of the Upper Quaternary period (Vernette 1977).The seabed is a mixture of fossil corals covered by mud from the Canal del Dique to the south of the bay and of other sediments s that arrive from the Magdalena River to the north, converging in the Bocagrande area (Thomas et al. 2005).
The surface sediments of Bocagrande consist mainly of fine silts and muds that arrived from the Canal del Dique, a channel that communicates the bay with the Magdalena River (Thomas et al. 2005).Wave and current dynamics suspend these sediments easily.This condition limits the visibility of optical sensors on the seabed, so there has been non-objective verification of man-made structures in that area.

Methodology
The first step in work was to gather primary bibliographic and cartographic sources, as well as bathymetric data obtained over the course of three separate season or projects.The Naval Oceanographic Office (NAVOCEANO) made the first sounding with the demonstration echosounder in 2014.The second took place during the execution of the 2016 nautical cartography plan, which saw the drawing of the first high-resolution chart of the Bay of Cartagena.The third survey was carried out in 2019 as part of the Submerged Cultural Heritage project.In this last campaign, 95% of the submerged area of "La Escollera" was mapped in high resolution.
The high-frequency beam survey system consists of an echosounder equipped with a transducer capable of emitting 512 acoustic pulses simultaneously, generating a range of beams, hence the term "multibeam."Acoustic waves hit the bottom, bounce back, and their echo is detected by the transducer.Since the speed of sound traveling through water is known, the time that passes between the emission of the fan of beams and the return of the signal determines the depth that each beam has reached.The transducer can separate the depth measured by each acoustic wave from the angle at which it was emitted.This process is repeated up to 100 times per second.CARIS HIPS and SIPS® software were used for data processing (Fig. 3).
The generated bathymetric surfaces allowed for a comparison between the structures in historical charts and the forms found in situ with this technology.The high-resolution multibeam system surveyed 1914 m of the 2102 m length of the Escollera wall found under the sea.No data was collected in areas adjacent to the coast due to breaking waves and the shallow depth (Fig. 3).
The analysis of old cartographic information allows us to identify certain features and correlate that historical data with present day charts and survey data.This study reviewed maps from 1757, 1769, 1777, 1779, and 1788, which detail the construction of La Escollera in the Bocagrande sector of the bay of Cartagena.
Georeferencing is a process that assigns coordinates to an image (plane) that lacks them or that employs another coordinate system through use of a Geographic Information System (GIS).Points on a base image need to be identified that can be correlated with coordinates on the image to be georeferenced.We then locate the same element in Fig. 3 The bathymetric surface of the 2021 DIMAR-CIOH.La Escollera appears as the continuous shallow (around 2 m deep) wall between El Laguito and Tierrabomba both images, obtain its coordinates in the base image, and attribute those coordinates to the same element or point on the image.These points are known as "control" points.
A preliminary study must therefore be conducted to locate points that have not changed over time and are easily identifiable.These can be unique buildings such as docks, structures, walls, or features on the ground.Sometimes this process can be done in stages (e.g., Cerezo 2011).Because the information obtained with these techniques can easily be integrated with other data, they are ideal to be managed with GIS, sometimes as the tool from which to manage and display the information from the different geophysical sensors, and other times to carry out analyses and predictions based on historical-archaeological and oceanographic information (Ryan and McGrath 2008).
For the georeferencing of the plans and charts (Table 1), a satellite image from 2021 was used, in which the following control points were captured: Geo1 (Baluarte de Santo Domingo), Geo2 (Santa Clara), Geo3 (Fort of San Sebastián de Pastelillo), Geo4 (Fort of Manzanillo), Geo 5 (Fort of Santa Cruz de Castillo Grande) and on La Escollera (Geo6, Geo7, and Geo 8), as indicated in Fig. 4.
Aerial and satellite images from the National Hydrographic Service were used to search for anomalies in the shallow waters near the position of La Escollera de Bocagrande.The aerial photograph was taken from the National Geographic Service of Colombia in 1961.The Landsat/Copernicus satellite image was taken in 2003.The fieldwork plan was based on satellite and photographic information to carry out the bathymetric survey of the study area.To conduct the survey, we used boats of the General Maritime Directorate with personnel and equipment from the National Hydrographic Service, part of the Center for Oceanographic and Hydrographic Research of the Caribbean.
The high-resolution bathymetric survey was conducted with a Kongsberg 2040C multibeam system configured to search for objects on the seafloor, with a maximum resolution of 3 cm.For the survey, a frequency of 400 kHz was selected, making it possible to generate highly detailed bathymetric surfaces.A DGPS system with satellite differential correction with centimeter precision was used for positioning.These data were processed and analyzed using CARIS Hips and Sips software version 10.4, which allowed the production of a 3D bathymetric surface, with a pixel size of 0.05 m.Coverage in shallow waters was extended to where the depth no longer allowed for safe navigation.
In order to record the anomalies in the study area ground truthing using scuba diving in the Bocagrande sector.These inspections were carried out in September 2020.

Results and Discussion
The From north to south, the submerged part of the Escollera includes a relatively wide section, which runs between 54 and 59 m along the first 501 m oriented SSW (Fig. 6).From that distance, the structure becomes less than half of that width, around 20 m, for the next 395 m (Fig. 7).Where it was constructed over the ruins of Fort San Matías in the center of La Boca, the Bocagrande Breakwater reaches a width of around 35 m in the section that covers the old eastern bastions of the Fort, over a length of about 200 m (Fig. 8).It then narrows again to 517 m between the ruins of the Forts San Matías and Santangel, where it has a width of only about 16 m-the breakwater's narrowest stretch-and widens again to between 35 and 44 m before reaching the shallow part of the island of Tierrabomba (Fig. 9).
The general structure of the Escollera remains almost intact.The apparent lack of landslides and deformations account for the constant homogeneity in the construction and the height from the seabed observed in the satellite image and the superimposition of the bathymetric surface (Fig. 10).
The map of Lorenzo de Solis from 18 August 1757, shown in Fig. 11, places the position of the ruins of San Matías castle (E) in the center of Bocagrande, at a distance of "77 boxes of 50 Castilian varas", or approximately 1,278.2 m (1 vara = 0.83 m) between "Punta de la Raya" (A)-where La Escollera was born on "Playa Grande"-and of "65 boxes" or 1,128.8m from the "Ruins of Fort" (Santangel) in the northern tip of Tierrabomba (F).
Andrade-Amaya et al. ( 2022) established the coordinates of the starting point of the Bocagrande Escollera in Punta Raya.With geographical reference on the Escollera, it was possible to georeference the Solis plan (1757) and compare it with the current coastline clarifying the position of Fort San Matías.The distances in Castilian yards measured on the map also help to confirm that the ruins of Fort San Matías are located 1412 m from the current point of El Laguito (Fig. 12).This assessment is corroborated by another map made in 1788 in Cadiz, indicating a strong, square bastion on the strip of sand that joined Bocagrande and Tierrabomba, halfway between both points (Fig. 13).
The Fort and some of its features is also referred to by Segovia Salas (1992): The last re-foundation of the San Matías takes place in 1602.But the Fort, a small quadrangle with bastions at the corners, is mortally wounded.Already in 1599, its transfer to Punta Judío (where the Naval Club stands today) had been proposed.The location of the San Matías, on a shifting ledge of the Tierrabomba peninsula, was considered too inconvenient."Tierra Floxa" ["loose ground"], as Bocagrande was then known, where the changing regime of breezes and tides left [San Matías Castle] uselessly anchored on land from time to time, exposed it to assault and far from its normal maritime mission.
The plans of Engineer Antonio de Arebalo from 1777 show the structure of the Bocagrande Escollera (Figs. 14,15).This submarine wall was intended to block the entrance through the mouth between Bocagrande and Tierrabomba Island, 2 km further south.The detailed plan of the Bocagrande "Malecón" describe this section of La Escollera, the position of the ruins of San Matías Fort and the Santangel Platform (Figs. 14,15).The position of San Matías Fort also appears in the map in Muñoz (1821), entitled "Plan of the Bocagrande Escollera between the beach of that name and Tierrabomba to close the Entrance to the Bay of Cartagena."In this plan, the structure under study is described with its own name as it appears in the legend of the plan in Fig. 7 where it is noted "…Profile No. 2 Cut by line 3. 4. that represent the Solid of Stones in which the   16).
The transformation of the coastline of the exterior coast of the Bocagrande peninsula from 1759 to the present day has been significant because the structure of La Escollera blocked the littoral sand drift adding 41 hectares of sediments (Andrade-Amaya et al. 2022), which caused the section that remains between its origin and the current "Espolón Iribarren" to remain buried in the sand (Fig. 17).The rapid development of the buildings and the subsequent stabilization of the Laguito sandbar mean that the identification of these structures can only be carried out through a formal and comparative exercise between the old charts and with new remote sensing techniques.For this 1 3 purpose, a process was carried out to ensure the exact comparison between the old maps and the current morphology that allowed the objective interpretation of the different features of the structures that were visible.

Analysis of Satellite Images and Aerial Photography
Vertical aerial images and satellite images free of cloud cover were studied.One aerial photograph from 1961 and one image from 2013, available on Google Earth, had enough water transparency to see the Escollera as a whole and made it possible to observe the square appearance of the ruins of the submerged Fort (Fig. 18).

Analysis of Bathymetric information
The bathymetric surface data shows one conspicuous anomaly outside of La Escollera, which is due to its extension and placement in the external part of the Bay that was unrelated to the structure.Contextual analysis determined that it was unlikely that this feature was caused by landslides, overturning or accidental material removal during the construction of La Escollera (Fig. 19).
A more detailed analysis of the bathymetric surface reveals that the anomaly has a square shape with arrow-shaped structures in its corners, with 82 m between them and a very well-defined internal square of 24 × 24 m, surrounded by a square form with a thickness of 17 m.Designs in the form of walls are also apparent.The submerged Escollera stands out, 2 m deep (in red).The ruins of the fort are at 6 m, and the squared layout and points at the corners can be identified (Fig. 20).
To identify whether the contours of the anomaly were consistent with a fort, we compared the dimensions of the anomaly with those of the Castillo de San Marcos, a Spanish fortress located in St. Augustine, Florida, United States.The comparison shows that the anomaly corresponds to the model of a bastioned fort or castle (see Fig. 21), indicating that they are consistent in shape and dimensions.

Visual Inspections by Diving
Inspections of the study area by divers were undertaken to confirm the evidence from primary sources and the multibeam data.This inspection recorded ashlars, wood, and structures of brick and mortar (Fig. 22).
The images in Fig. 22 correspond to the attributes related to structures of the ruins of the Fort.It is possible to distinguish what appear to be bricks joined together to form a structure.These are remains of masonry constructions, ashlars, wood and other materials.
There are two principal factors behind the sinking of the land where the Fort was established.The main factor is the fact that the sand that constituted the spit was brought by coastal drift.The abrupt shutdown of this coastal drift stopped feeding material to the sandbar, causing a net material deficit.The second factor is that the Cartagena Bay area is known to be subsiding at the rate of 3 mm per year, apparently due to gas escape and mud vulcanism, which is common throughout the area (e.g., Mora-Paez et al. 2018).
However, despite the centuries that have passed and the circumstances in which the ground has subsided in the area, the external wall in the southern ruins of the fort remains visible, which spans around 17 m in length and stands 4 m tall (Fig. 23).general feature is a trench of about 1 m deep and 5 m wide that has formed along the inner side running along the entire structure of the Escollera (Fig. 24).
A view of three profiles over the Escollera with different constructed thicknesses is shown in Fig. 25.The bathymetric sections in parts of the Escollera are compared with oblique 3D views of the same sectors.
It has also been possible to analyze aspects and construction details described in Antonio de Arebalo's the original 1773 plans to close the opening of Bocagrande, which identify, for example, the largest width and thinnest areas and to what type of construction they belong.Three aspects are of interest regarding the width of the Escollera.The Escollera is at its widest along the 500 m stretch closest to the city, where it measures around 50 m.In addition, it is wider than the rest of the structure when it reaches the depths around Tierrabomba Island, which reveals the construction method used to carry out the work, perhaps considering the concentration of waves at these points.The rest of the work is thinner and constructed with two lines of parallel boxes facing each other.
A special construction was required for the Escollera to pass over the remains of the eastern bastions of Fort San Matías, forming a section of the Escollera that was abnormally wider than the rest of the route.The variations in the thickness of the Escollera depended on three factors.In terms of construction, it was made wider in the first part of the Bocagrande area and the shallow area before reaching Tierrabomba; another constructive form related to the passage of the Escollera over the ruins of Fort San Matías and a slender thickness in the rest of the Escollera.
The construction methodology at the beginning of the submerged zone consists of two lines of caissons approximately 50 m apart.Later, in the intermediate zone, just one line of caissons was built, interrupted only by the remains of San Matías before continuing, as shown in Fig. 25.The continuous trench on the entire inner side of the Escollera (on the bay side) can be explained by the turbulent movement of water as it passes over the structure, creating vertical vortices that accumulate sediment on the outside and scour fine sediments in the internal part without causing any danger to the stability of this structure.

The Position of the Santangel Platform
Due to the shallow depth at the southern end of La Escollera, the Santangel Platform does not appear in the multibeam survey.However, we want to highlight the Santangel platform   The appearance of the submerged part of La Escollera de Bocagrande is in excellent condition, there is no evidence of landslides, and the rocks of its construction are covered by a thin layer of marine encrustations.This speaks to the dedication and talent of the eighteenthcentury engineers who decided to construct an underwater wall of more than 2.5 km to defend the city, which is still in operation and excellent condition with no further maintenance.

The Position of Punta Icacos
It appears that Punta de Icacos, the name given to the tip of the sandbar between Bocagrande and Tierrabomba, has continually changed its position.A 1769 map shows the tip reaching the San Matias ruins (Fig. 28a), while modern maps place it closer to Cartagena (Fig. 28b, c) including at the current location of the manmade lake and peninsula known as El Laguito (see Fig. 3), which was created in the 1960s.This argument explains the misplacement of the San Matias ruins in recent documents.

Conclusions
The bathymetric surface superposed over the georeferenced 1758 map confirms the anomaly corresponds to the ruins of Fort San Matías.The ruins are sunken in the middle of Bocagrande in the Bay of Cartagena de Indias.The bathymetric surface analyzed shows a square construction with bastioned corners that is consistent with the dimensions of the Castillo de San Marcos, corroborating this hypothesis.

Fig. 1 Fig. 2
Fig. 1 Bocagrande is seen at different times in history.The red arrow indicates the area where the terrain alternately appeared and disappeared until the construction of the Breakwater.a Diagram of the Bay of Cartagena de Indias (Blanco circa 1507?).b Map showing the capture of Cartagena by the English squadron commanded by Francis Drake in 1587 (de Bry 1599).c Plan of the Bay of Cartagena de Indias (circa 1626) (UKHO 2014) where it is observed that Bocagrande was closed by a continuous strip of sand and the Fort of San Matías dominated it in the center.d Drawing of the Bay of Cartagena (AGI 1637) showing an open navigation channel in the southern part of Bocagrande, defended by the Fort of Santangel.e A chart by Bouclet and Aubert (1793-1820); the arrow points to Bocagrande, closed at the time of the arrival of the Baron de Pointis Squadron in 1697.f This diagram of the siege of Cartagena de Indias made by the English squadron commanded by Admiral Edward Vernon in 1741 shows a small canal again in the sand bar that closes Bocagrande (Laws 1741).g Bathymetric surveys carried out in the bay of Cartagena de Indias upon the arrival of the company of engineers after the English attack (Arebalo 1764) found Bocagrande increasingly wide as compared with 1741 plans.h Plan of the layout of the Bocagrande Breakwater made by Arebalo (1769).It involved the construction of a continuous rock structure that closed the Bay to high-seas navigation through Bocagrande (Color figure online) total length of La Escollera is 2536 m.The northern end of the Bocagrande breakwater now lies below the modern day 5th and 6th Avenue in the Bocagrande neighborhood, just 33 m from the edge of the inner bay.A 622 m stretch of the breakwater lies buried under the beaches and part of the Bocagrande neighborhood, due to the accumulation of sediment that this project generated (Andrade-Amaya et al. 2022).Approximately 1914 m of the breakwater lies underwater, extending from the beaches of Bocagrande to the beginning of El Laguito (Gómez-Pretel and Carvajal-Diaz 2011), whose top is around 2 m deep on a sand base and silt that is about 6 m deep.The methodology describes the surveyed segment of la Escollera (Fig.5).

Fig. 5 3 Fig. 6
Fig. 5 The map of 1778 (SME 1995) superimposed on a Google Earth ® 2022 image in the Bocagrande area in Cartagena de Indias highlights the 622 m of buried structure.Underwater, the bathymetric surface shows the 1914 m from La Escollera (in red) along its submerged part to the shallowest area on Tierrabomba Island (Color figure online)

Fig. 8
Fig. 8 The Bocagrande Escollera over the ruins of Fort San Matías (discussed below) in the center of La Boca is about 35 m wide in the sector that covers the old eastern bastions of the Fort, for an extension of 200 m

Fig. 9
Fig. 9 The Bocagrande Escollera between the ruins of Forts San Matías and Santangel has a width of around 16 m-the breakwater's narrowest stretch.It then widens back out to between 35 and 44 m before reaching the shallow part of the island of Tierrabomba

Fig. 10
Fig. 10 On the left, The Escollera in the Google Earth® satellite image.Right, the previous image with the bathymetric surface on top, for comparison purposes

Fig. 13
Fig. 13 Plan of the Port and City of Cartagena de Indias made in Cádiz in 1788.(A) Enlargement of the plan defining Fort of San Matías (in the circle).This plan was kindly provided by the United Kingdom Hydrographic Office (UKHO)

Fig. 17
Fig. 17 Comparing the map from December 1759 with a satellite image from December 2022 allows us to observe the land that was gained in Bocagrande mainly due to the construction of the Breakwater (adapted from Andrade-Amaya et al. 2022)

Fig. 18
Fig. 18 Images of the Bocagrande area, in which the underwater jetty is visible.(A) Aerial photograph of the Geographic Service of Colombia "Agustín Codazzi" from 1961: on the right is the enlargement of the box in (A), identifying the outline of San Matías in the center of La Escollera.(B) Satellite image of Landsat/Copernicus from 2003.To the right, an enlargement of (B), outline San Matías

Fig. 19
Fig. 19 Analysis of the central sector of the Bocagrande Breakwater.(A) Bathymetric overlay on the 1769 plan.(B) Bathymetric surface detail over San Matías.(C) Detail of the georeferenced 1769 plan, highlighting the ruins of the bases of the San Matías Fort

Fig. 21
Fig. 21 Diagram of a bastioned fort superimposed on the bathymetric surface in the study area.(Right) Satellite image of the Fort San Marcos in St. Augustine, Florida, United States, Google Earth

Fig. 23
Fig. 23 The southern external wall of the Fort, about 17 m long.If the bottom is 6 m deep and the top of this wall is 2 m deep, this wall is about 4 m high beneath the sea

Fig. 24 (
Fig.24(Above) The bathymetric surface between Bocagrande and Fort San Matías.(Below) this surface seen from Tierrabomba looking towards Fort San Matías.The bathymetric surface shows La Escollera, built in 1778 (in orange and red colors) on the bottom of the bay (in green colors).The structure is a welldefined wall in good condition, with no apparent collapse.A ditch runs through the entire inner part of the Escollera.This trench is about 1 m deep and between 3 and 5 m wide

Fig. 25
Fig. 25Bathymetric sections of the Escollera.Sectors correspond to different constructive forms described in the plans of the time.The Q-Q′ section corresponds to the slenderest part of the construction using only a single box.The T-V section belongs to the provision made to cover the ruins of Fort San Matías.The S-R section corresponds to the part of the Escollera that was widened by placing two separate caissons and filling between them.In the middle of the Boca and interposed between the Escollera are the remains of the Fort San Matías

Fig. 26 (
Fig. 26 (A) The position of the Santangel Platform, enlarged (B) at the southern end of the Breakwater in the 1631 map (AGI 1631).(C) The architecture and description of the Santangel Platform in a plan of 1617 (AGI 1617)

Fig. 28 a
Fig. 28 a Fragment of the 19 July 1769 map, signed by Antonio de Arebalo on the area of Bocagrande, which shows the position that Punta de Icacos would have had at that time, as well as the ruins of San Matías.Punta de Icacos and San Matías in b Segovia (1992), c in Porto (2016)

Table 1
Plans, charts, and images used for the comparison and georeferencing of the submerged Escollera and the Bocagrande sector