For the archaeologist on land, the main dangers come from land developers. The marine archaeologist, however, must beware of the sea as they try to preserve shipwrecks from the pounding waves. Each fragile wreck is a unique time capsule of our past: encrusted cannons, ancient wine bottles and other historical treasures. Treasures that the sea is loath to give up. This is the story of underwater archaeology.
Those involved in a shipwreck may lose everything in the catastrophe, including perhaps their lives. But the remains of sunken vessels and their contents, even if broken up and scattered, provide future generations with unique windows into the past. Each site represents a moment frozen in time, and every item recovered from it is part of a vast three-dimensional archaeological jigsaw. Archaeologists know they can never complete it, but by analysing the evidence they find, and fitting it into the jigsaw's framework, they come ever closer to their goal of constructing a true picture of the ship before it became a wreck. Each has a special story to tell, and is therefore a fragile and irreplaceable microcosm of its times.
Diving on sunken ships has happened throughout history. Alexander the Great is reputed to have gone down in a diving bell, while Roman urinatores (free divers) apparently salvaged cargo from a wreck 20m deep off the south of France.
The aqualung opened the underwater world to diving adventurers like Hans Hass and Jacques Cousteau in the 1940s. Although many ancient wrecks were found, little thought was given to their archaeological significance. It was not until 1960 that a young archaeologist, George Bass, teamed up with explorer Peter Throckmorton to investigate a Bronze Age shipwreck at Cape Gelidonya in Turkey. Bass learned to dive and showed that archaeology could be done underwater to the same standards as on land. Bass's team is now excavating a Bronze Age shipwreck of the 14th century BC at Uluburun off Turkey.Since then techniques have been refined, but the principles remain unchanged. Many classical wrecks have been investigated in the Mediterranean, such as the Roman wine carrier with 6000 amphoras off Madrague de Giens near Marseilles.
Underwater archaeologists working around the British Isles have investigated several wrecks of the Spanish Armada, such as the Girona off Antrim and La Trinidad Valencera off Donegal. The biggest project has been the excavation and raising of Henry VIII's battleship, Mary Rose which sank in the Solent in 1545. The largely intact hull of the Dutch East Indiaman 'Amsterdam' was found off the English town of Hastings. The boat had been swallowed into the sand when she had beached there in 1748. The wreck has been the focus of investigations for 30 years now. Current projects include the Swan, a small Cromwellian warship, which was lost off the west of Scotland in 1653.
Notice to visitors at a wreck site
Well-preserved wrecks abound in the Baltic, where low salinity and cold water slow down decay. These include the Vasa, which sank on her maiden voyage in 1628 and was raised from Stockholm Harbour in 1961. Another is the Kronan, which blew up and sank during a battle with the Dutch in 1676. Not all wrecks are now under water. Many have been found in reclaimed parts of Holland's Zuider Zee, after the land has been drained. At Skuldelev in Denmark a group of five Viking ships, sunk to form a blockage at the entrance to a fjord.
Although they have to work in a hostile environment, underwater archaeologists follow the same principles as their counterparts on land. A site must first be surveyed. This is done with tape-measures and grids using simple geometry, supplemented by photography, video, and geophysics. The aim is to produce an analytical map so that the wreck can be assessed. Often this will enable archaeologists to record the site and interpret its significance without having to disturb it. Only in exceptional cases are historic shipwrecks excavated, and their contents recovered.
Well-preserved wrecks have usually reached a state of equilibrium with their environments. Having survived for centuries in this state, they will probably remain in much the same condition for centuries to come. But if they are disturbed, the stability which has protected them will be broken. Archaeologists normally excavate shipwrecks only if they are under some kind of threat - a shift in the environmental balance, perhaps, or plans to dredge a harbour entrance where historic sites are known to lie. An example of this kind of 'rescue' excavation is provided by the Cromwellian warship Swan wrecked in 1653 off Mull, which is now threatened by seabed erosion.
Very occasionally archaeologists excavate wrecks which are not under threat to recover them and their contents for public exhibition. Examples of these include the Swedish warship Vasa at Stockholm (1628) and the Mary Rose at Portsmouth (1545). In such cases the damage to their archaeological integrity is offset by careful records made by the archaeologists, the conservation of the ships and their contents, and subsequent publication and display.
Parts of the hull tell the marine archaeologist how the ship was designed and built. Tool-marks reveal woodworking techniques used by the shipwrights. Fragments of rigging and rope - sometimes even pieces of sails - help to show how the ship was operated by its crew. Other finds throw light on activities such as navigation, medical care, everyday crafts, food storage, cooking, and domestic routines. If the ship was a merchant vessel parts of its cargo may have survived, perhaps revealing where it came from. Warships contain evidence of the weapons they carried. Above all, items of clothing and personal possessions bring us into immediate contact with people who lived and died long ago.