HMHS Britannic was the third ship in the Olympic class ocean liner built by White Star Line, and the sister ship of RMS Olympic and RMS Titanic. Due to World War I, she was never used as a transatlantic passenger liner and ended being used as a hospital ship in 1915. On 21 Nov 1916, the ship was sunk likely by an underwater mine off the Greek island of Kea. Of the 1.066 people aboard, 30 lost their lives. Since it sank in shallow waters, the wreck is easily accessible to divers and numerous expeditions have taken place. Although the wreck is in Greek waters, the British have designated it as a war grave and both governments must approve expeditions to the wreck. In 1996 the wreck itself was sold to author Simon Mills whose desire is to leave it as is.

Sources:
1. HMHS Britannic(Wikipedia)
2. Nova Online: Titanic’s Lost Sister