The Mongolian Empire, rulers over the continent, attacked Japan twice in the middle of the Kamakura era, the "Mongolian Invasions." In the second invasion, known as the Battle of Koan (1281), a huge flotilla of 4,400 ships and 140,000 men was dispatched, only to be destroyed by a typhoon in the bay of Imari. Until now, surveys revealed no trace of the Mongolian invasion ships. In 2011 however, a major survey was undertaken using the latest detection instruments, finally excavating the ships, which were buried under the sea floor mud though still in conditions allowing for reconstruction.