Lake Nemi is a flooded volcanic crater in the hills south of Rome, a little over a mile across, with no river running into it and none running out. Somewhere in the four years of his reign, between 37 and 41 AD, the emperor Caligula had two ships built on it, one of them 230 feet long and the other 240, which is about the size of a modern car ferry, but neither of them could go anywhere at all.
This is the first thing to understand about the Nemi ships: they were not built to sail, because on a lake a mile wide there is nowhere to sail to. What they were built for was the lake itself, which the Romans called the speculum Dianae, the mirror of Diana, after the sanctuary of Diana Nemorensis that stood on its shore and drew pilgrims from all over Latium.
