North Sea, or German Ocean, a southern extension of the Arctic Ocean. Its northern boundary would be represented by a line drawn from the Shetland Islands to the opposite coast of Norway, and its southern boundary is the Strait of Dover; W. it is bounded by Great Britain, and E. by Norway, Denmark, Germany, Holland, and Belgium. With the Atlantic it communicates through the Strait of Dover and the English Channel on the south, and on the north by the Pentland Firth and the channel between the Orkney and Shetland Islands; and with the Baltic by the Skagerrack and Cat-tegat. The North Sea is over 600 miles in length and about 400 miles in maximum width, and its area exceeds 160,000 sq. m. By far the greater proportion of this area is less than 100 fathoms in depth, the only part where deeper water - 360 fathoms - is found being off the coast of Norway. The sea is very shallow towards the south and east, and the coasts in this direction are low and flat, being in some places below the level of the sea, whereas to the north and west, where the water is deeper, the sea-coast is high, and the deep ' Norwegian Gully' is faced by the high and bold cliffs of Norway. The sea-bottom is very irregular, a number of banks running across from the Yorkshire coast towards the Skagerrack, the most important of which is the Dogger Bank (q.v.), and there are also depressions like the Silver Pit; off the low-lying coasts of Holland, Belgium, and Britain there are numerous shoals and sandbanks formed of the materials brought down by the rivers. The North Sea receives many rivers, the principal being the Thames, Ouse, Humber, Tyne, Tweed, Forth, and Tay, the Scheldt, Rhine, Weser, and Elbe. The North Sea has been from the earliest times one of the most important highways of the world, and is surrounded by some of the most prosperous commercial nations. Its fisheries are among the greatest in existence, providing employment for thousands of fishermen.