A submerged city is not excavated simply by finding attractive ruins and lifting objects from the seabed. Archaeologists first work out what remains, where it lies and how it relates to the landscape. Water can hide streets, walls, harbours and environmental deposits beneath sediment. Mapping those relationships is essential because an artefact removed without a record loses much of the information that made it useful.
Survey before excavation
Teams combine historical research with bathymetry, sonar, magnetometry, sediment sampling and direct observation by divers or remotely operated equipment. These methods answer different questions. Bathymetry maps seabed shape; sonar can reveal buried or raised features; magnetometry may detect metal; sediment cores help reconstruct changing coastlines. Remote sensing narrows the search area, while archaeologists inspect and verify possible structures on site.
At Thonis-Heracleion in Egypt's Aboukir Bay, survey work helped locate and map a settlement beneath the water. The site lies offshore under layers of sediment, so researchers used survey before excavating selected areas. Evidence includes architecture, religious objects, harbours and everyday material. None of these finds makes the site a complete snapshot: buildings collapsed, shorelines shifted and material accumulated over centuries.
Context is part of the find
Underwater excavation uses careful recording because visibility can be limited and currents or sediment can disturb a trench. Archaeologists establish grids, photograph and draw features, measure depths, label finds and record exact positions. A stone block's location and relation to a wall can help identify a building phase. A vessel's distribution may show where it broke or how cargo moved. Without such records, objects become disconnected from the activities and environments they once belonged to.
Not every submerged site is a city, and not every underwater ruin sank in one disaster. Sea-level rise, earthquakes, gradual subsidence, storms, river changes and human activity can contribute to submergence. Researchers compare local geology, building phases, historical accounts and artefact distributions. Some sites cover settlements that once stood on dry land; others preserve harbours or wrecks that were always part of a maritime landscape.
Conservation begins underwater
Waterlogged wood, metal, ceramics and stone respond differently when exposed to air. An artefact can deteriorate quickly if raised without a conservation plan, controlled transport and suitable storage. Archaeologists may therefore document a feature and leave it in place. The UNESCO Convention treats preservation in situ as the first option, with recovery considered when it serves research or protection and can be followed by proper conservation.
Raising an object is not the endpoint of every investigation. A small, selected sample may answer a research question, while mapping and photography preserve the wider site. Public access also needs management: responsible diving can help people understand underwater heritage, but uncontrolled contact can damage fragile remains.
Reconstructing a submerged settlement
Researchers combine site plans with environmental history and material culture to understand how a settlement worked. A harbour basin, temple district and residential quarter leave different patterns, but erosion and later construction blur boundaries. Thonis-Heracleion and other sites are interpreted through many seasons of survey and excavation, not one dramatic discovery.
Underwater archaeology is a sequence of decisions about observation, recording, sampling and care. The goal is not to remove the largest possible number of objects. It is to preserve evidence and explain how people lived with changing coastlines and waterways. Thorough documentation lets even a feature left on the seabed contribute to a clearer account of the past.
Frequently asked questions
How do archaeologists find submerged cities?
They combine historical research with seabed mapping, sonar, geophysical surveys, sediment sampling and inspection. Survey identifies promising areas before targeted excavation.
Why not bring every artefact to the surface?
Objects can deteriorate when removed from water, and lifting them without documentation destroys context. UNESCO guidance prioritizes preservation in place when appropriate.
How did Thonis-Heracleion become submerged?
It lies beneath the water and sediment of Egypt's Aboukir Bay. Its submergence involved changing coastal and geological conditions over time, not necessarily one simple event.
Can sonar identify a whole city by itself?
No. Sonar and other remote methods help map possible features, but archaeologists inspect and interpret them with excavation records, geology and artefacts.