The White Sea is a tributary of the Arctic Ocean. It forms the northern coast of the European part of Russia and covers an area of about 90,000 km². Its average depth is 67 m; the maximum depth extent is over 350 m. Since the White Sea is almost completely surrounded by land and has only a comparatively narrow connection to the open sea, it is also called the marginal sea. The sovereign rights to the use of the waters lie with Russia. The harbors are often iced over until May. Channels lead from the White Sea to the Black Sea and the Caspian Sea.