Nemi Ships: Caligula's Floating Pleasure Palaces

How Caligula's Floating Pleasure Palaces Were Found and Lost Again. 

The Nemi Ships (wiki) - so named because they were discovered in Lake Nemi, south of Rome: The ships were destroyed during WWII, when the purpose-built museum in which they were held caught fire.

From Imperium Romanum:

"Why were the ships unique? The ships were monumental and full of amazing technology. They were over 70 meters long and 24 wide, a system of supplying hot and cold water (pipes with the name of the emperor), bathrooms and baths. Everything in marbles, gold and ivory. Undoubtedly they served the Emperor’s private pleasures – a large living room, kitchen or bedroom indicate that the ships were treated as mobile palaces. 

People queuing in 1932 to see one of the two ships
 of Roman Emperor Caligula, which were recovered
 from Lake Nemi, Italy, on Mussolini's orders.

They contained Archimedes’ screws (the oldest known to us), an extremely rare anchor of admiralty at that time; piston pumps (discovered again only in the Middle Ages). Both had a system of moving sculptures (modelled on the theatre) with a ball bearing system."

The ships were destroyed by fire during World War II on the night of 31 May 1944. Several shells of the United States Army hit near the museum around 8 pm, causing little damage but forcing the German artillery to leave the area. There are conflicting views on whether the bomb started the fire or the Germans set as they left. 

Much more on this at Wikipedia.

An artistic depiction of a Nemi Ship by CM Knight-Smith, c 1906