A shipwreck is the remains of a ship that has wrecked, which are found either beached on land or sunken to the bottom of a body of water. Shipwrecking may be deliberate or accidental. UNESCO estimates that worldwide over 3 million shipwrecks, some thousands of years old, lie on seabeds.
Historic wrecks are attractive to maritime archaeologists because they preserve historical information: for example, studying the wreck of Mary Rose revealed information about seafaring, warfare, and life in the 16th century. Military wrecks, caused by a skirmish at sea, are studied to find details about the historic event; they reveal much about the battle that occurred. Discoveries of treasure ships, often from the period of European colonisation, which sank in remote locations leaving few living witnesses, such as the Batavia, do occur but only very infrequently. Some contemporary wrecks, such as the oil tankers Prestige or Erika, are of interest primarily because of their potential harm to the environment. Other contemporary wrecks are scuttled in order to spur reef growth, such as Adolphus Busch and the Ocean Freeze. Wrecks like Adolphus Busch and historic wrecks such as SS Thistlegorm are of interest to recreational divers that dive to shipwrecks because they are interesting to explore, provide large habitats for many types of marine life, and have an interesting history.
Very few shipwrecks are as famous as the catastrophic Titanic, Britannic, Lusitania, Estonia, Empress of Ireland, Andrea Doria, or Costa Concordia. There are also thousands of wrecks that were not lost at sea but have been abandoned or sunk. These abandoned, or derelict ships are typically smaller craft, such as fishing vessels. They may pose a hazard to navigation and may be removed by port authorities. There are more than 3 million wrecks on the ocean floor, the United Nations estimates.
Poor design, improperly stowed cargo, navigation and other human errors leading to collisions (with another ship, the shoreline, an iceberg, etc.), bad weather, fire, and other causes can lead to accidental sinkings. Intentional reasons for sinking a ship include forming an artificial reef; due to warfare, piracy, mutiny or sabotage; as part of target practice; or to remove a menace to navigation. A ship can be also used as breakwater structure.
Many factors determine the state of preservation of a wreck:
The above-mentioned, especially the stratification (silt/sand sediments piled up on the shipwrecks) and the damages caused by marine creatures is better described as "stratification and contamination" of shipwrecks. The stratification not only creates another challenge for marine archaeology but also a challenge to its primary state, the state that it had when it sank.
Stratification includes several different types of sand and/or silt, as well as tumulus and encrustations. These "sediments" are tightly linked to the type of currents, depth, and the type of water (salinity, pH, etc.), which implies any chemical reactions that would lead to affecting the hypothetical/possible main cargo (such as wine, olive oil, spices, etc.).
Besides this geological phenomenon, wrecks also face the damage of marine creatures that create a home out of them; primarily being octopuses and crustaceans. These creatures affect the primary state because they move, or break, any parts of the shipwreck that are in their way, thereby affecting the original condition of amphorae, for example, or any other hollow places. Finally, in addition to the slight or severe destruction marine animals can create, there are also "external" contaminants, such as modern-day commodities, or contemporary pollution in bodies of water, that as well severely affect shipwrecks by changing the chemical structures, or even destroying or devastating even more of what is left of a specific ship.
All the above offers great challenges to the marine archaeologist when attempting to bind the pieces of a certain shipwreck together. Despite these challenges, if the information retrieved does not appear to be sufficient, or a poor preservation is achieved, authors like J.A. Parker claim that it is the historical value of the shipwreck that counts, as well as any slight piece of information and/or evidence that is acquired.
Exposed wooden components decay quickly. Often the only wooden parts of ships that remain after a century are those that were buried in silt or sand soon after the sinking. An example of this is the Mary Rose.
Steel and iron, depending on their thickness, may retain the ship's structure for decades. As corrosion takes place, sometimes helped by tides and weather, the structure collapses. Thick ferrous objects such as cannons, steam boilers or the pressure vessel of a submarine often survive well underwater in spite of corrosion.
Shipwrecks in some freshwater lakes, such as the Great Lakes of North America, have remained intact with little degradation. In some sea areas, most notably in Gulf of Bothnia and Gulf of Finland, salinity is very low, and centuries-old wrecks have been preserved in reasonable condition. However, bacteria found in fresh water cause the wood on ships to rot more quickly than in seawater unless it's deprived of oxygen. Two shipwrecks, the USS Hamilton and the USS Scourge, have been at the bottom of Lake Ontario since they sunk during a violent storm on August 8, 1813, during the War of 1812. They are in "remarkably good" condition. 
Wrecks typically decay rapidly when in seawater. There are several reasons for this:
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An important factor in the condition of the wreck is the level of destruction at the time of the loss or shortly afterwards due to the nature of the loss, salvage or later demolition.
Examples of severe destruction at the time of loss are:
After the loss the vessel's owners may attempt to salvage valuable parts of the ship and/or its cargo; this operation can cause further damage.
Shipwrecks in shallow water near busy shipping lanes are often demolished or removed to reduce the danger to other vessels. On charts, some wreck symbols have a line under the symbol with a depth mark, which indicates the water depth above the wreck.
On the seabed, wrecks are slowly broken up by the forces of wave action caused by the weather and currents caused by tides. Also more highly oxygenated water, which promotes corrosion, reduces the strength of ferrous structural materials of the ship. Deeper wrecks are likely to be protected by less exposure to water movement and by lower levels of oxygen in water.
Extreme cold (such as in a glacial-fed lake), Arctic waters, the Great Lakes, etc. slow the degradation of organic ship materials. Decay, corrosion and marine encrustation are inhibited or largelly absent in cold waters.
Often, attempts are made to salvage recently wrecked ships to recover the whole or part of the ship, its cargo, or its equipment. A good example of this was the scuttling and subsequent salvage of the German High Seas Fleet at Scapa Flow in the 1920s. The unauthorized salvage of wrecks is called wrecking.
Shipwreck law determines important legal questions regarding wrecks, perhaps the most important question being the question of ownership. Legally wrecks are divided into wreccum maris (material washed ashore after a shipwreck) and adventurae maris (material still at sea); although some legal systems treat the two categories differently, others treat them the same.
Wrecks are often considered separately from their cargo. For example, in the English case of the Lusitania  QB 384 it was accepted that the remains of the vessel itself were owned by the insurance underwriters who had paid out on the vessel as a total loss by virtue of the law of subrogation (who subsequently sold their rights), but that the property aboard the wreck still belonged to its original owners (or their descendants).
Military wrecks, however, remain under the jurisdiction–and hence protection–of the government that lost the ship, or that government's successor. Hence, a German U-boat from World War II still technically belongs to the German government, even though the Third Reich is long-defunct. Many military wrecks are also protected by virtue of their being war graves.
However, many legal systems allow the rights of salvors to override the rights of the original owners of a wreck or its cargo. As a general rule, non-historic civilian shipwrecks are considered fair game for salvage. Under international maritime law, for shipwrecks of a certain age, the original owner may have lost all claim to the cargo. Anyone who finds the wreck can then file a salvage claim on it and place a lien on the vessel, and subsequently mount a salvage operation (see Finders, keepers). The State of North Carolina claims "all photographs, video recordings, or other documentary materials of a derelict vessel or shipwreck or its contents, relics, artifacts, or historic materials in the custody of any agency of North Carolina government or its subdivisions" to be its property.
Some countries assert claims to all wrecks within their territorial waters, irrespective of the interest of the original owner or the salvor. Wartime wrecks have different legal considerations, as they are often considered prizes of war, and therefore owned by the Navy that sank them.
Some legal systems regard a wreck (and/or its cargo) to be abandoned if no attempt is made to salvage them within a certain period of time. English law has usually resisted this notion (encouraged by an extremely large maritime insurance industry, which asserts claims in respect of shipwrecks which it has paid claims on), but it has been accepted to a greater or lesser degree in an Australian case and in a Norwegian case.
The American courts have been inconsistent between states and at Federal level. Under Danish law, all shipwrecks over 150 years old belong to the state if no owner can be found. In Spain, wrecks vest in the state if not salvaged within 3 years. In Finland, all property on board shipwrecks over 100 years old vests in the state.
The British Protection of Wrecks Act, enacted to protect historic wrecks, controls access to wrecks such as Cattewater Wreck which can only be visited or investigated under licence. The British Protection of Military Remains Act 1986 also restricts access to wrecks which are sensitive as war graves. The Protection of Military Remains Act in some cases creates a blanket ban on all diving; for other wrecks divers may visit provided they do not touch, interfere with or penetrate the wreck. In the United States, shipwrecks in state waters are regulated by the Abandoned Shipwrecks Act of 1987. This act is much more lenient in allowing more open access to the shipwrecks.
Following the beaching of the MSC Napoli, as a result of severe damage incurred during European storm Kyrill, there was confusion in the press and by the authorities about whether people could be prevented from helping themselves to the flotsam which was washed up on the beaches at Branscombe. Many people took advantage of the confusion and helped themselves to the cargo. This included many BMW motorbikes and empty wine casks as well as bags of disposable nappies (diapers). The legal position under the Merchant Shipping Act 1995 is that any such finds and recovery must be reported within 28 days to the Receiver of Wreck. Failure to do so is an offence under the Merchant Shipping Act and can result in a criminal record for theft by finding. After several days, the police and Receiver of Wreck, in conjunction with the landowner and the contracted salvors, established a cordon to prevent access to the beach. A similar situation occurred after the wreck of the MV Cita in 1997.
Historic wrecks (often but not always defined as being more than 50 years of age) are often protected from pillaging and looting through national laws protecting cultural heritage. Internationally they may be protected by a State ratifying the Unesco Convention on the Protection of the Underwater Cultural Heritage. In this case pillaging is not allowed. One such example is the Queen Anne's Revenge which is undergoing archaeological recovery by the North Carolina Department of Cultural Resources near Beaufort Inlet, NC. The infamous pirate Blackbeard's flagship was discovered by Intersal, Inc., a private research firm, in 1996.
An important international convention aiming at the protection of underwater cultural heritage (including shipwrecks) is the Convention on the Protection of the Underwater Cultural Heritage. The 2001 UNESCO Convention on the Protection of the Underwater Cultural Heritage represents the international community's response to the increasing looting and destruction of underwater cultural heritage. It forms part of a group of UNESCO standard setting instruments regarding the domain of cultural heritage, encompassing seven conventions adopted by UNESCO Member States, which constitute a coherent and complementary body guaranteeing a complete protection of all forms of cultural heritage.
The UNESCO 2001 Convention is an international treaty aimed exclusively at the protection of underwater cultural heritage and the facilitation of international cooperation in this regard. It does not change sovereignty rights of States or regulate the ownership of wrecks or submerged ruins.
In 2011, the most valuable cargo of a sunken shipwreck was identified near the western edge of the Celtic Sea. This World War II era sinking of the SS Gairsoppa led to a treasure almost three miles deep.
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