The HMHS Britannic (sister to the ill-fated RMS Titanic) was sailing off the coast of Kea on November 21, 1916 when she was rocked by an explosion - she had been struck by a mine. World War I had been underway for the past two years, and the ship was loaded with wounded soldiers from the Middle Eastern theatre.
Although built with more safety features than her older sister, the ship went down in about 55 minutes.
Several portholes had been opened at the time of the explosion, which resulted in the ship sinking so quickly.I can't remember where I found this, but it's a rather neat image of Britannic laying on the bottom of the Kea Channel in Greece. I believe it was taken during Dr. Robert Ballard's expedition to the wreck site in 1995.
This image was taken by diver Antonello Paone. Note the size of the person beside the massive propeller.