Probing World War I maritime mystery.

For 80 years, the circumstances concerning the sinking of the Britannic have been the subject of debate among military and maritime historians. The luxury liner, a sister to the ill-fated Titanic, had been converted to a hospital ship during World War I. On Nov. 21, 1916, during its sixth voyage to pick up wounded troops from the Dardanelles, Britannic either was torpedoed or hit a mine and sank within 55 minutes of the explosion.

In an effort to solve the much-disputed mystery of the sinking, oceanographer Robert D. Ballard, who had discovered the wreck of the Titanic, mounted an expedition to the Aegean Sea, where Britanic's hull rested 400 feet below the surface. Working with two remote-operated vehicles with undersea cameras and a small nuclear-power submarine with a Navy crew, Ballard explored the wreckage. Unfortunately, the expedition...