Underwater archaeology is the surveying and excavation of maritime features such as shipwrecks and sunken harbours. It is also used to rediscover archaeological features found in other bodies of water such as submerged lakeside settlements.
Its prominence as a discipline has increased over the last couple of decades as increased technology has allowed more sites to become accessible for discovery and investigation with over 1000 shipwrecks discovered from the classical and medieval periods.
Invaluable information about the past has been retrieved. Whereas dry sites generally deal with abandoned settlements and objects, underwater archaeological features such as the shipwrecks of the Mary Rose and Titanic preserve snapshots of everyday life. Underwater archaeology has also been used to build up a more complete picture of ancient trading and trade routes.
The techniques used underwater are broadly the same as those used on land, with adaptations to accommodate the differences of excavating sites and conserving artefacts found under water.
Underwater Surveying
Various observational and geophysical survey techniques are used to identify and examine underwater features:
· Divers. Local divers familiar with an area of seabed are still used widely to help discover underwater features. This basic surveying method has been used to discover many of the shipwrecks in the Mediterranean.
· Mini subs and submersibles. Modern technology now allows access to parts of the sea bed too deep for divers. Mini subs and submersibles have been used to pinpoint and explore features in deep water such as the Titanic.
· Proton Magnetometry. Iron and steel distort the earth’s magnetic field. These distortions can be picked up by a proton magnetometer dragged across the sea bed and used to pinpoint metal objects such as steel hulls and cannons.
· Sonar scanning. Sound waters are transmitted in a fan shape over the seabed, producing an image of any surface anomalies. This method led to the discovery of the exact location of the Mary Rose.
· Sub-bottom profiling. Used to identify subsurface features, sound waves are transmitted to the sea bed. They are bounced back by any objects below the seabed, identifying their positions.
Underwater Excavation and Recording
Although scuba diving gear increases the amount of time underwater excavators can spend working on a site, it is not possible for any one person to remain under the surface indefinitely, making excavation underwater more complex than that on land. Specialist equipment helps the process.
· Airlifts. Sediment and material around vessels, structures and objects is removed from the site using specialised tubes which suck away the unwanted material like a vacuum cleaner.
· Lift bags. These specially designed metal baskets attached to balloons are used to transport objects safely to the surface.
Methods of recording the site and positions of objects are also adapted to suit underwater conditions. Recording and drawing straight onto paper is impossible. Instead, the outlines of features are traced onto a plastic sheet held over the area to be recorded with a measuring grid in place to plot positions.
Preserving Finds
Many items are perfectly preserved underwater. There are also particular challenges in maintaining them once they reach the surface. Chlorides, sulphates and carbonates are leeched from metal objects to form hard coats which preserve them below the water. Once above surface, these salts react with the air to form an acid that eats away at the object. By using electrolysis, this process can be halted. The object is placed in a chemical solution on a metal grill. A weak electronic current is then passed through the solution, drawing off the salts and so preventing the decay of the metal.
Sources
Archaeology: An Introduction (1995) Kevin Greene. B T Batsford Limited. London
Archaeology: Theories, Methods and Practice (1994) Colin Renfrew and Paul Bahn.Thames and Hudson.
Natasha Sheldon - A writer since 2000, Natasha Sheldon holds a BA Hons in ancient history and archaeology and MA in ancient history and historiography.
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