Shipwreck

A shipwreck is the wreckage of a ship that is located either beached on land or sunken to the bottom of a body of water. Shipwrecking may be intentional or unintentional. There were approximately three million shipwrecks worldwide as of January 1999, according to Angela Croome, a science writer and author who specialized in the history of underwater archaeology ^[1] (an estimate rapidly endorsed by UNESCO^[2][3] and other organizations^[4]).

When a ship's crew has died or abandoned the ship, and the ship has remained adrift but unsunk, they are instead referred to as ghost ships.

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 Batavia, do occur as well. Some contemporary wrecks, such as the oil tanker 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 Ocean Freeze. Many contemporary and historic wrecks, such as 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.

Well-known shipwrecks include the catastrophic Titanic, MV Doña Paz, Britannic, Lusitania, Estonia, Empress of Ireland, Andrea Doria, Endurance 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^[5] and may be removed by port authorities.^[6]

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 sinking. Intentional reasons for sinking a ship include: intending to form an artificial reef; destruction due to warfare, piracy, mutiny or sabotage; using the vessel for target practice; or removing a menace to navigation. A ship can be also used as breakwater structure.

Many factors determine the state of preservation of a wreck:

• the ship's construction materials
• the wreck becoming covered in sand or silt
• the salinity of the water the wreck is in
• the level of destruction involved in the ship's loss
• whether the components or cargo of the wreck were salvaged
• whether the wreck was demolished to clear a navigable channel
• the depth of water at the wreck site
• the strength of tidal currents or wave action at the wreck site
• the exposure to surface weather conditions at the wreck site
• the presence of marine life that consume the ship's fabric
• temperature
• the acidity (or pH), and other chemical characteristics of the water at the site

The above - 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 determine its primary state, i.e. the state that it was in when it sank.

Stratification includes several different types of sand and 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 affect potential 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 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 the artifacts on and around the wreck at Pickles Reef and the over-lapping wrecks at the Molasses Reef Wreck, or contemporary pollution in bodies of water, that severely affect shipwrecks by changing the chemical structures, or further damaging what is left of a specific ship.

Despite these challenges, if the information retrieved does not appear to be sufficient,^{[further explanation needed]} or a poor preservation is achieved, authors like J.A. Parker claim that it is the historical value of the shipwreck^{[further explanation needed]} that counts as well as any slight piece of information or evidence that is acquired.^[7]

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 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.

Propellers, condensers, hinges and port holes were often made from non-ferrous metals such as brass and phosphor bronze, which do not corrode easily.

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 is deprived of oxygen.^[8] Two shipwrecks, USS Hamilton and USS Scourge, have been at the bottom of Lake Ontario since they sunk during a viol