Underwater archaeology is archaeology practiced underwater. As with all other branches of archaeology, it evolved from its roots in pre-history and in the classical era to include sites from the historical and industrial eras.
Its acceptance has been a relatively late development due to the difficulties of accessing and working underwater sites, and because the application of archaeology to underwater sites initially emerged from the skills and tools developed by shipwreck salvagers. As a result, underwater archaeology initially struggled to establish itself as actual archaeological research. This changed when universities began teaching the subject and a theoretical and practical base for the sub-discipline was firmly established in the late 1980s.
Underwater archaeology now has a number of branches including, maritime archaeology: the scientifically based study of past human life, behaviors and cultures and their activities in, on, around and (lately) under the sea, estuaries and rivers. This is most often effected using the physical remains found in, around or under salt or fresh water or buried beneath water-logged sediment. In recent years, the study of submerged WWII sites and of submerged aircraft in the form of underwater aviation archaeology have also emerged as bona fide activity.
Though often mistaken as such, underwater archaeology is not restricted to the study of shipwrecks. Changes in sea level because of local seismic events such as the earthquakes that devastated Port Royal and Alexandria or more widespread climatic changes on a continental scale mean that some sites of human occupation that were once on dry land are now submerged. At the end of the last ice age, the North Sea was a great plain, and anthropological material, as well as the remains of animals such as mammoths, are sometimes recovered by trawlers. Also, because human societies have always made use of water, sometimes the remains of structures that these societies built underwater still exist (such as the foundations of crannogs, bridges and harbors) when traces on dry land have been lost. As a result, underwater archaeological sites cover a vast range including: submerged indigenous sites and places where people once lived or visited that have been subsequently covered by water due to rising sea levels; wells, cenotes, wrecks (shipwrecks; aircraft); the remains of structures created in water (such as crannogs, bridges or harbors); other port-related structures; refuse or debris sites where people disposed of their waste, garbage and other items, such as ships, aircraft, munitions and machinery, by dumping into the water.
Underwater archaeology is often complementary to archaeological research on terrestrial sites because the two are often linked by many and various elements including geographic, social, political, economic and other considerations. As a result, a study of an archaeological landscape can involve a multidisciplinary approach requiring the inclusion of many specialists from a variety of disciplines including prehistory, historical archaeology, maritime archaeology, and anthropology. There are many examples. One is the wreck of the VOC ship Zuytdorp lost in 1711 on the coast of Western Australia, where there remains considerable speculation that some of the crew survived and, after establishing themselves on shore, intermixed with indigenous tribes from the area. The archaeological signature at this site also now extends into the interaction between indigenous people and the European pastoralists who entered the area in the mid-19th century.
Research potential There are many reasons why underwater archaeology can make a significant contribution to our knowledge of the past. In the shipwreck field alone, individual shipwrecks can be of significant historical importance either because of the magnitude of loss of life (such as the Titanic) or circumstances of loss (Housatonic was the first vessel in history sunk by an enemy submarine). Shipwrecks such as Mary Rose can also be important for archaeology because they can form a kind of accidental time capsule, preserving an assemblage of human artifacts at the moment in time when the ship was lost.
Sometimes it is not the wrecking of the ship that is important, but the fact that we have access to the remains of it, especially where the vessel was of major importance and significance in the history of science and engineering (or warfare), due to being the first of its type of vessel. The development of submarines, for example, can be traced via underwater archaeological research, via the Hunley, which was the first submarine to sink an enemy ship (Hunley also had unique construction details not found in previous vessels and was one of the few historic warships ever raised intact); the Resurgam II, the first powered submarine; and Holland 5, which provides insight into the development of submarines in the British Navy.
UNESCO Convention All traces of human existence underwater which are one hundred years old or more are protected by the UNESCO Convention on the Protection of the Underwater Cultural Heritage. This convention aims at preventing the destruction or loss of historic and cultural information and looting. It helps states parties to protect their underwater cultural heritage with an international legal framework.UNESCO, Convention on the protection of the Underwater Cultural Heritage On the basis of the recommendations defined in the above-mentioned UNESCO Convention various European projects have been funded such as the CoMAS project for in situ conservation planning of underwater archaeological artefacts.*
Challenges Underwater sites are inevitably difficult to access, and more hazardous, compared with working on dry land. In order to access the site directly, diving equipment and diving skills are necessary. The depths that can be accessed by divers, and the length of time available at depths, are limited. For deep sites beyond the reach of divers, submersibles or remote sensing equipment are needed.
For a marine site, while some form of working platform (typically a boat or ship) is often needed, shore-based activities are common. Notwithstanding, underwater archaeology is a field plagued by logistics problems. A working platform for underwater archaeology needs to be equipped to provide for the delivery of air for example, recompression and medical facilities, or specialist remote sensing equipment, analysis of archaeological results, support for activities being undertaken in the water, storage of supplies, facilities for conservation for any items recovered from the water, as well as accommodation for workers. Equipment used for archaeological investigation, including water dredge and airlifts create additional hazards and logistics issues. Moreover, marine sites may be subject to strong tidal flows or poor weather which mean that the site is only accessible for a limited amount of time. Some marine creatures also pose a threat to diver safety.
Underwater sites are often dynamic, that is they are subject to movement by currents, surf, storm damage or tidal flows. Structures may be unexpectedly uncovered, or buried beneath sediments. Over time, exposed structures will be eroded, broken up and scattered. The dynamic nature of the environment may make in-situ conservation infeasible, especially as exposed organics, such as the wood of a shipwreck, are likely to be consumed by marine organisms such as piddocks. In addition, underwater sites can be chemically active, with the result that iron can be leached from metal structures to form concretions. The original metal will then be left in a fragile state. Artifacts recovered from underwater sites need special care.
Visibility may be poor, because of sediments or algae in the water and lack of light penetration. This means that survey techniques that work well on land (such as triangulation), generally can not be used effectively under water.
In addition it can be difficult to allow access to the results of the archaeological research as underwater sites do not provide good outreach possibilities or access for the general public. Work has been done to bridge this difficulty through the use of the World Wide Web for webcasting projects, or dedicated virtual reality systems that allow users to perform a virtual diving into an interactive 3D reconstruction of the underwater archaeological site. An example is the excavation of the Queen Anne's Revenge and the QAR DiveLive program, a live interactive virtual field trip to the wrecksite.
Techniques
Although specialized techniques and tools have been developed to address the challenges of working under water, the archaeological goals and process are essentially the same as in any other context. Investigating an underwater site however, is likely to take longer and be more costly than an equivalent terrestrial one.
An important aspect of project design is likely to be managing the logistics of operating from a boat and of managing diving operations. The depth of water over the site, and whether access is constrained by tides, currents and adverse weather conditions will create substantial constraints on the techniques that can feasibly be used and the amount of investigation that can be carried out for a given cost or in a set timescale. Many of the most carefully investigated sites, including the Mary Rose have relied substantially on avocational archaeologists working over a considerable period of time.
As with archaeology on land, some techniques are essentially manual, using simple equipment (generally relying on the efforts of one or more scuba divers), while others use advanced technology and more complex logistics (for example requiring a large support vessel, with equipment handling cranes, underwater communication and computer visualization).
Position fixing Knowing the location of an archaeological site is fundamental to being able to study it. In the open sea there are no landmarks, so position fixing is generally achieved using GPS. Historically, sites within sight of the shore would have been located using transects. A site may also be located by visually surveying some form of marker (such as a buoy) from two known (mapped) points on land. The depth of water at a site can be determined from charts or by using the depth sounding sonar equipment that is standard equipment on ships. Such sonar can often be used to locate an upstanding structure, such as a shipwreck, once GPS has placed the research vessel in approximately the right location.
Site survey
The type of survey required depends on the information that is needed to resolve archaeological questions, but most sites will need at least some form of topographical survey and a site plan showing the locations of artifacts and other archaeological material, where samples were taken and where different types of archaeological investigation were carried out. Environmental assessment of archaeological sites will also require that environmental conditions (water chemistry, dynamic properties) as well as the natural organisms present on the site are recorded. For shipwrecks, particularly post-industrial age shipwrecks, pollution threats from wreck material may need to be investigated and recorded.