When the Titanic, the pride of the White Star Line, sank on its first voyage in one of the most famous sea disaster of all time, the twin ship Britannic was still in the shipyard. As soon as it was built, it was converted into a hospital ship and was sunk during its first year at sea in mysterious circumstances. Since November 1916, it has been 112 metres deep in the Aegean Sea. The riddle of the sinking of the ship is still unsolved: a mine or a torpedo from a submarine caused the disaster? If the latter conjecture is true, then why? Was it a war crime or did the Britannic violate the international laws by transporting war material? The skin-divers of the Calipso reached the Britannic wreck to carry out a series of investigations to solve its mystery.