The West Antarctic Ice Sheet (WAIS) is the sector of the Antarctic Ice Sheet covering West Antarctica. The WAIS is a marine ice sheet, as the majority of its ice is grounded well below sea level, on rock and sediment that would be the ocean floor if the ice were to retreat. The ice sheet first formed about 27 million years ago, likely fed partially by advancing glaciers of the older East Antarctic Ice Sheet . The WAIS is bounded by the Ross Ice Shelf, the Ronne Ice Shelf, and outlet glaciers that drain into the Amundsen Sea.