When the last ice sheets covered much of Asia, the sea level fell low enough for a land bridge to appear between Japan and the Asian mainland. This route was taken by hunter-gatherers from Asia who crossed into previously uninhabited Japan. By 10,000 BC the first pottery was produced in Japan and there was some cultivation. Rice was introduced, probably from Korea, by about 400 BC, and the use of metals around a century later, but agriculture and fixed settlements were confined to the south for a long period. During this time waves of migrants came from mainland Asia, bringing with them skills and technologies, including the Chinese characters for writing.