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.