The Bering Sea is named after a Danish navigator in the Russian Navy, who explored the area in 1741. It lies between mainland Asia and North America, and is bounded by the Aleutian Islands to the south and linked to the Arctic Ocean in the north by the narrow Bering Strait. There is a flow of cold Arctic water south through this strait, feeding a counterclockwise circulation. The main freshwater input is the Yukon River, which has deposited an extensive delta at its mouth. The Bering Sea is one of the world’s richest fisheries, helping Alaska account for about half of the total US fish and shellfish catch. Harbor seals and gray whales also take advantage of these productive waters. In contrast to the deep ocean basin beneath the southwestern half of the sea, the broad continental shelf in the northwest is very shallow. Much of this area formed a land bridge during the last ice age, when sea levels were up to 390 ft (120 m) lower than they are today. This route was ice-free for extended periods, allowing several species, including humans, to migrate from Asia to North America on foot for the first time.