Cetacean stranding, commonly known as beaching, is a phenomenon in which whales and dolphins strand themselves on land, usually on a beach. Beached whales often die due to dehydration, collapsing under their own weight, or drowning when high tide covers the blowhole. Cetacean stranding has occurred since before recorded history. Several explanations for why cetaceans strand themselves have been proposed, including changes in water temperatures, peculiarities of whales' echolocation in certain surroundings, and geomagnetic disturbances, but none have so far been universally accepted as a definitive reason for the behavior. However, a link between the mass beaching of beaked whales and use of mid-frequency active sonar has been found. Subsequently, whales that die due to stranding can decay and bloat to the point where they can easily explode, causing gas and their internal organs to fly out. Species Every year, up to 2,000 animals beach themselves. Although the majority of strandings result in death, they pose no threat to any species as a whole. Only about ten cetacean species frequently display mass beachings, with ten more rarely doing so. All frequently involved species are toothed whales (Odontoceti), rather than baleen whales (Mysticeti). These species share some characteristics which may explain why they beach. Body size does not normally affect the frequency, but both the animals' normal habitat and social organization do appear to influence their chances of coming ashore in large numbers. Odontocetes that normally inhabit deep waters and live in large, tightly knit groups are the most susceptible. This includes the sperm whale, oceanic dolphins, usually pilot and killer whales, and a few beaked whale species. The most common species to strand in the United Kingdom is the harbour porpoise; the common dolphin (Delphinus delphis) is second-most common, and after that long-finned pilot whales (Globicephala melas). Solitary species naturally do not strand en masse. Cetaceans that spend most of their time in shallow, coastal waters almost never mass strand. Causes Strandings can be grouped into several types. The most obvious distinction is between single and multiple strandings. Many theories, some of them controversial, have been proposed to explain beaching, but the question remains unresolved. Natural deaths at sea The carcasses of deceased cetaceans are likely to float to the surface at some point; during this time, currents or winds may carry them to a coastline. Since thousands of cetaceans die every year, many become stranded posthumously. Most carcasses never