Today shipwrecks and underwater ruins are coming under increasing threat. While professional equipment and a high-level of training are necessary to undertake underwater excavations, this heritage is no longer beyond the reach of treasure hunters. In addition to dispersal, recovered objects face also the risk of destruction due to the lack of conservation.
In 1942 and 1943, Jacques-Yves Cousteau and Emile Gagnan invented the aqualung, thereby making it possible to reach wrecks located at greater sea depths. Since the beginning of the twenty-first century, open circuit divers have been able to descend to a depth of up to 100 metres, and to even more with the discovery of new gas mixtures. In 1989, the Japanese research submarine Shinkai 6.500 dived even to 6,527 metres below sea level in the Trench off Sanriku, Japan. In 1995, the Japanese unmanned probe Kaiko descended to the record depth of 10,911 metres.
Increased accessibility allows archaeologists to better understand underwater cultural heritage and the public to enjoy it. However, it can also lead to pillaging and damage.
Threat of Looting and Dispersal
Many archaeological sites under water are subject to heavy pillage. Their exploitation and the sale of the objects found is reminiscent of events that took place a hundred years ago on many archaeological sites on land.
More than 345 large shipwrecks have been commercially exploited on a big scale in the last thirty years, with up to 500,000 objects recovered and sold per wreck, and the ship’s hull left destroyed. The grey amount of pilfering and looting is however much higher, as irresponsible leisure divers increasingly damage wrecks and sites. Research has shown for instance that only a small minority of wrecks located in diveable depths along the coasts of the Mediterranean remain untouched.
Famous wrecks, damaged or destroyed by commercial exploitation, include the Geldermalsen, the Nuestra Señora de Atocha, and the Cirebon and Belitung wrecks.
Insufficient Legal Protection
Without adequate legal protection, underwater sites can be easily exploited by treasure hunters or be negatively impacted by industrial activities. Even when some legal protection exists, gaps in legislation and State sovereignty rights enable treasure hunters to pursue their activities and exploit artefacts for mere commercial purposes in disregard for the loss to humanity and science.
UNESCO took action by elaborating the 2001 Convention on the Protection of the Underwater Cultural Heritage.
Construction and Trawling
Fishing nets and trawling devices may in some instances get caught on underwater cultural sites. Consequently, some elements may be moved and dragged over the seabed thus damaging or exposing the site. To avoid this, buoys may be installed and protective parks established. However, this is obviously only possible for known sites.
Infrastructural works such as the construction of ports, dredging, pipelines and oil or mineral extraction may also damage sites. This was the case of the Oranjemund wreck in Namibia and the Mardi Gras wreck off the coast of Louisiana; German archaeologists undertook special efforts to protect the shipwreck barrier off Greifswald.
While such damages can not be avoided entirely, they should be mitigated as far as possible and archaeological inspection should be mandatory before dredging or construction operations. This is already being done in many States and the cooperation between archaeological services and construction companies has proven fruitful and positive for both sides.
Submerged archaeological sites are not only threatened by human activities, but also by changes in the environment caused by earthquakes, storms, temperature variations, changing currents or costal erosion. Usually, a site that lies buried under sediment reaches a stable anaerobic state with low levels of oxygen and light that reduces the process of material degradation caused by chemical, physical and biological factors. The following threats may however occur:
Physical threats: Erosion, abrasion and scoring: Harbor works, dams and other infrastructural constructions may change the course of currents and produce erosion of the seabed, exposing underwater cultural heritage that was previously buried. Abnormal waves produced by underwater earthquakes can also disrupt the equilibrium of a site, removing the sediment over it and exposing it to water or bacteria. Many sites are also exposed to air due to sea level changes and may in consequence be destroyed.
Biological threats: Biological impact of fungi, bacteria and woodborers: Bacteria-induced decay threatens wood in marine sites and even more so sites in fresh water. The attack of woodborers, or Toredo Navalis, also poses a considerable biological threat to submerged wood elements. This shipworm can destroy wood within a few months. As an example, the Baltic Sea was until recently free of Toredo Navalis as are the waters of the Arctic and Antarctic. This is one of the main reasons why shipwrecks are so well preserved in their waters.
Chemical threats: Chemical processes can disturb the integrity of a submerged archaeological site, as for instance the corrosion of iron and other metals, which augments with the availability of oxygen on a site. Also, bacteria activity on wood can form minerals, such as pyrite, which is detrimental to wood because of the low PH3 which causes hydrolysis of the cellulose. Several important ships recovered from the seabed such as the Vasa, the Mary Rose or the Batavia have suffered damage as a result of such processes.