Hokkaido is the second largest main island of Japan, and the largest and northernmost prefecture. The Tsugaru Strait separates Hokkaido from Honshu. It was formerly known as Ezo, Yezo, Yeso, or Yesso. The two islands are connected by the undersea railway Seikan Tunnel. The largest city on Hokkaido is its capital, Sapporo, which is also its only ordinance-designated city.