I have just returned from Scapa Flow.
At the end of the First World War, the whole German High Seas fleet was interned at Scapa Flow, a British navy base in the Orkney islands north of Scotland. On 21 July 1919, unaware that the WWI armistice had been extended, and fearing a new start of hostilities, Admiral Ludwig von Reuter decided that the ships should not be seized by Britain. He sent the order to open the seacocks and sink the vessels. In all, 74 ships went down in one afternoon in what was the biggest mass-scutling in history.
Most of the vessels have now been salvaged for scrap metal, but some battleships can still be visited, such as the 24,000-tonnes Kronprinz Wilhelm and Markgraf. Several light cruisers (5,000 tonnes) like the Desden, Brummer, Karlsruhe and Coln also offer very interesting dives.
Although there is a UK section on this site, Scapa Flow deserves its own album!