Underwater archaeology: reading cities and ships beneath the water

Underwater archaeology is not simply land archaeology performed with diving gear. Water changes what survives, how a site can be reached and what evidence an excavator can safely remove. A harbour may preserve stone walls and cargo in place; a wreck may retain a hull because it was rapidly buried in sediment; another site may have been scattered by currents, dredging or later salvage. The first task is therefore to understand the whole deposit before treating any object as a discovery in isolation.

Finding a site before excavating it

Researchers commonly begin with remote sensing and geophysical survey. Sonar can map the shape of the seabed; magnetometers can detect some buried metal; sub-bottom instruments can trace layers beneath sediment. Divers or remotely operated vehicles then inspect selected targets and record what is visible. The sequence matters: a survey can show a feature that deserves investigation, but it does not automatically identify a temple, a ship or a date.

At Thonis-Heracleion in Egypt’s Aboukir Bay, geophysical and geological work helped reconstruct a submerged part of the ancient Canopic region and locate concentrations of archaeological remains. Excavations and inscriptions then helped identify the city and its harbour setting. The site was not recovered by finding one treasure. It became legible through the relationship among a former Nile channel, harbour basins, temple precincts, objects and written evidence.

That distinction corrects a common oversimplification: Thonis-Heracleion did not simply vanish in a single earthquake on a neatly known date. The Bay of Aboukir contains a low-lying delta landscape affected by sediment, subsidence and changes in water level. Geological instability and human land use may have acted together over time. The specific sequence and timing remain questions for research, so a confident one-event explanation goes beyond the evidence.

Harbours are not the same as shipwrecks

A submerged harbour can preserve quays, basins, building foundations and channels, while a wreck records the final position of one vessel and its cargo. Even a wreck is rarely a sealed time capsule in the simple sense: wood can be eaten by organisms, metal can corrode, cargo can shift, and the wreck can be disturbed after sinking. Archaeologists record the position of timbers and objects because spatial relationships can reveal loading, repairs, breakage and later movement.

Cold, low-oxygen water sometimes slows biological decay, which is one reason wooden vessels survive in parts of the Baltic. But preservation depends on local chemistry, burial, salinity and disturbance. A ship found in one sea cannot be used as a universal model for every wreck. Its construction and cargo also need independent dating and comparison before historians can infer where it sailed or who owned it.

Underwater sites are vulnerable to looting and commercial salvage. The UNESCO Convention on the Protection of the Underwater Cultural Heritage treats preservation in place as the preferred first option and calls for research that respects the archaeological context. Removing an object may protect it from immediate damage in some circumstances, but uncontrolled recovery destroys information about where it lay and how the site formed.

From a mapped feature to a historical claim

Good interpretation separates observation from explanation. “A stone line continues beneath the sediment” is an observation. “It formed the edge of a harbour basin” is an interpretation supported by its shape and setting. “A sudden disaster drowned the whole city” is a wider causal claim that requires geological and chronological evidence beyond the wall itself.

The same caution applies to submerged Alexandria. Archaeologists investigate parts of the ancient waterfront and harbour landscape, including structures associated with the city’s royal quarter. A trace of a monumental building can contribute to the history of the Pharos area, but popular descriptions sometimes label every underwater ruin as the lighthouse itself. Identification needs diagnostic architecture, location and a sound excavation record.

Submerged remains can tell us how people built ports, moved goods and adapted to changing coasts. To see how land-based deposits are read in a different setting, compare the methods used to find buried cities; for the role of everyday deposits, see what ancient rubbish reveals. The shared principle is context: the location and formation of evidence often matter as much as the object.