ATHENS, Greece (AP) _ A solitary note pealed from the ship's alarm bell, minutes before the funnels collapsed onto the upper deck and the liner plunged to the depths of the Aegean.

Rescue ships answering the SOS call arrived expecting to find the stricken British ocean liner. Instead, they saw only a line of lifeboats making their way slowly towards the island of Kea.

Four years after the Titanic sank in the icy North Atlantic waters, its even larger sister ship, the Britannic, was swallowed by the sea.

Dubbed the world's largest shipwreck currently on the seabed, the Britannic has lain in relative obscurity since 1916.

Now, an international team of professional divers is preparing to explore the wreck beginning Nov. 1 and hopes to shed some light on the mysterious circumstances surrounding its sinking during World War I.

Still under construction when the Titanic struck an iceberg on its maiden voyage in April 1912, the Britannic's design was changed to correct defects in what was supposed to be its unsinkable sister.

The Britannic was originally built as one of three luxury ocean liners destined for the Southampton to New York route, but the British government requisitioned the liner before its maiden voyage to act as a hospital ship.

The Britannic went down in the Kea channel on Nov. 21, 1916, while on its fourth trip from Southampton to the island of Lesbos to pick up casualties. Twenty-eight people perished; 1,106 survived.

Historians still are unsure whether the ship ran into a mine or was hit by a torpedo.

Also a mystery is why the Britannic sank so quickly, taking only 57 minutes to go under water despite its apparently improved design. The Titanic took three times as long to sink.

``This will be the first-ever true survey of the wreck,'' says British diver Kevin Gurr, one of the organizers of ``Project Britannic.''

The ship was discovered lying at a depth of about 400 feet by an expedition led by underwater explorer Jacques Cousteau in the late 1970s. The technology of the time allowed divers only a very brief stay at such depths _ about 10 to 15 minutes at the most, Gurr explains.

In the mid-1980s, Robert Ballard, the president of the Institute for Exploration in Mystic, Conn., led the only other expedition to visit the wreck. Using an underwater robot, he photographed the wreck _ which is apparently in very good condition, with only one hole in its hull from the explosion.

But the robot was unable to enter far into the wreck to see whether all the automatic watertight doors were shut at the time of the sinking.

Individual divers will have more flexibility to probe the wreck, said organizer Alexandros Sotiriou, who has already carried out a preliminary dive to pinpoint the ship's location.

The group will be using up-to-date techniques, including underwater propulsion vehicles and a mixture of gases _ oxygen, helium and nitrogen _ to increase the time divers will be able to spend at the wreck, Gurr said.

The great depth increases the risks faced by the divers, and they will have to undergo about four hours of decompression before they are able to surface at the end of every dive.

``We may go into the wreck, but it will depend on the circumstances when we're there,'' Sotiriou said.