Hokkaidō, formerly known as Ezo, Yezo, Yeso, or Yesso, is Japan's second largest island and the largest, northernmost of its 47 prefectural-level subdivisions. The Tsugaru Strait separates Hokkaidō from Honshū, although the two islands are connected by the underwater Seikan Tunnel. The largest city on Hokkaidō is its capital, Sapporo, which is also its only ordinance-designated city. Archeologists theorize that Hokkaidō was settled by Ainu, Gilyak, and Oroke 20000 years ago. The Nihon Shoki is often said to be the first mention of Hokkaidō in recorded history. According to the text, Abe no Hirafu led a large navy and army to northern areas from 658 to 660 and came into contact with the Mishihase and Emishi. One of the places Hirafu went to was called Watarishima, which is often believed to be present-day Hokkaidō. However, many theories exist in relation to the details of this event, including the location of Watarishima and the common belief that the Emishi in Watarishima were the ancestors of the present-day Ainu people. During the Nara and Heian periods, people in Hokkaidō conducted trade with Dewa Province, an outpost of the Japanese central government. From the medieval ages, the people in Hokkaidō began to be called Ezo. Around the same time Hokkaidō came to be called Ezochi or Ezogashima. The Ezo mainly relied upon hunting and fishing and obtained rice and iron through trade with the Japanese.