When Japanese forces invaded Korea in 1592, their armies stormed ashore with terrifying speed. Castles fell, cities burned, and the road to the Korean capital lay open. At sea, however, the invaders met a problem they had never seen before: Admiral Yi Sun-sin. Yi commanded a small, disciplined navy that relied on powerful, sturdy Korean warships rather than sheer numbers. At the heart of his fleet sailed a bizarre vessel, low and squat, its curved roof studded with iron spikes and its prow carve