Approximately one-third of the Arctic (1) _______ is underlain by continental shelf, (2) _______ includes a broad shelf north of Eurasia and the narrower shelves of North America and Greenland. Seaward of the continental shelves (3) _______ the Arctic Basin proper, which is subdivided (4) _______ a set of three parallel ridges and four basins (also (5) _______ as deeps). These features were discovered and explored (6) _______ in the late 1940s. The Lomonosov Ridge, the major ridge, cuts the Arctic Basin (7) _______ in half, extending as a submarine bridge 1,800 km (1,100 mi) (8) _______ Siberia to the northwestern tip of Greenland. Parallel (9) _______ it are two shorter ridges: the Alpha Ridge on the North American (10) _______, defining the Canada and Makarov basins, and the Arctic Mid-Ocean Ridge on the Eurasian side, defining the Nansen and Amundsen basins. The average depth of the Arctic Ocean is only 1,300 m (4,300 ft) because of the vast shallow expanses on the continental shelves. The deepest point in the Arctic Ocean is 5,450 m (17,880 ft).