Anti-submarine sonar may have killed a group of whales found dead in the Hebrides in one of Britain's most unusual strandings, scientists believe. Five Cuvier's beaked whales, a species rarely seen in British waters, were discovered on beaches in the Western Isles on succeeding days in February. Another animal from a related species was discovered at the same time. Experts consider such a multiple stranding to be highly abnormal. The main suspect in the case is sonar, as it is known that beaked whales are highly sensitive to the powerful sound waves used by all the world's navies to locate underwater objects such as submarines. Groups of beaked whales have been killed, with sonar suspected as the direct cause, several times in recent years; well-documented incidents include anti-submarine exercises in Greece in 1996, the Bahamas in 2000 and the Canary Islands in 2002. Britain's Whale and Dolphin Conservation Society has now submitted a Freedom of Information request to the Ministry of Defence over the Hebridean strandings. The 21 species of beaked whale include some of the world's most rarely seen mammals; they are also the deepest-diving air-breathing animals. A Cuvier's beaked whale set the record for a deep dive two years ago: 1,899 metres, or 6,230ft, beneath the surface, holding its breath for an astonishing 85 minutes. The animals use these deep dives to forage, but when sonar gets involved, their remarkable habit may be their undoing. One theory is that the whales are so distressed by the intensely loud sound waves that they return too quickly to the surface, and in doing so, fatally suffer the bends" - the formation of nitrogen bubbles in the blood which can kill human divers.