Why the sea floor needs special methods
The “secrets” of Thonis-Heraklion are not hidden treasure so much as information locked in mud. The site lies in shallow water in Abu Qir Bay, where visibility is often poor and where a thick layer of soft sediment covers most of what survives. A diver can swim over a large temple wall without seeing it. For that reason the project, led by Franck Goddio and his institute with Egyptian authorities, relied on instruments first and divers second.
The basic principles are the same as on land, covered in the general article on archaeological methods: record where each object lies, work out its relationship to the others, and avoid destroying evidence while taking it out. Water adds problems of its own, among them limited time on the bottom, drifting silt, and the difficulty of fixing positions.
Step one: searching with instruments
Before any excavation, a research vessel towed or carried sensors over a grid pattern in the bay.
- Side-scan sonar sends sound pulses sideways and builds an image of the seabed, showing mounds, walls and wrecks that rise above it.
- Magnetometers detect small changes in the magnetic field, which can reveal buried iron, fired clay or large masses of stone. Anchors, ceramics and some building materials produce distinctive signals.
- Sub-bottom profilers use sound that penetrates the sediment and show layers below the sea floor, helping locate buried structures and former channels.
The resulting maps showed anomalies, which were then checked individually. This is a similar logic to LiDAR survey on land, where a remote method suggests places for closer examination, although sound and magnetism replace laser light.
Step two: reading the ground
Geoarchaeology is central to the project. Cores, narrow tubes of sediment drilled from the sea floor, show how the landscape changed: where the shore stood, where channels flowed, and where soft clay was thick. Researchers study the layers to see whether land sank, how sea level moved, and when sudden events such as flooding took place. These results are what make it possible to discuss why the city sank, a question dealt with in a separate article.
Dating relies on several lines of evidence. Pottery and coins are used for typological dates, in the manner of pottery seriation. Organic samples, such as wood or charcoal, can be tested by radiocarbon dating, though dates from a marine environment need careful interpretation. Inscriptions, such as royal names, give another check.
Step three: diving and excavation
Once a target is chosen, divers lay out a fixed reference grid and measure positions from it. Their basic tools are simple: tape measures, hand fans or trowels, numbered tags and cameras. To remove sediment they often use an airlift, a tube that draws mud and sand upwards, powered by compressed air, and discharges it away from the dig so that the work area does not silt up. Because divers can stay down only for limited periods at these depths, excavation advances slowly, and each object is photographed and recorded before it is moved.
Large objects present their own challenge. A multi-ton statue must be strapped, lifted with floats or cranes, and kept wet and stable as it comes to the surface. Waterlogged stone and wood are fragile once exposed to air, so conservators desalinate and stabilise them, a process that can take a long time.
Step four: recording and interpreting
Everything is placed on a digital map. Positioning systems tie diver measurements to the survey grid, and photographs are often combined into three-dimensional models. The result is a plan of the temple area, canals and harbours that can be compared with the written sources. Specialists then study each category of find, from ceramics to metalwork, and publish them in reports.
What is still limited
Even with modern equipment, a large part of the site has not been excavated in detail, and any plan is provisional. Poor visibility, strong sediment cover and the sheer size of the area mean that interpretations will continue to change. Some researchers also point out that underwater archaeology is expensive, so choices about which areas to excavate are always partly practical, and excavation is generally balanced against the option of leaving remains undisturbed.
Why the method matters
The Thonis-Heraklion project is a useful example because it shows how the physical and historical sciences work together. Without the geophysical survey the city would still be a name in Herodotus, and without careful recording the finds would be objects with no context. The same toolkit is now used on harbours and shipwrecks around the world.