The continents are entirely surrounded by what is called the "continental shelf" which slopes very gradually to a depth of about 500 feet. This shelf is built up of sediment (沉积的) which has been brought down to the sea by the continental rivers. Where the land near the coast is low and flat, the continental shelf may extend thirty, forty or even more miles from the coast; where mountain ranges run parallel to the sea and near to it (as on the east coast of Africa) the shelf is narrowed to only a mile or two.
From the edge of the shelf the sea-bed slopes sharply downwards so that the depth of water from the suce rapidly becomes greater, down to perhaps twelve thousands feet, to the floor of the ocean, the "abyss". No less than three-quarters of the waters of the sea lie above the abyss and only one-quarter covers all the continental shelves and slopes in the world.
The abyss (深渊), the great ocean floor under two miles or more of water, is not flat like a plain. On the contrary, it is more mountainous than the land. Its ranges are higher and its valleys deeper than those on the continents. There are, as yet, no maps of the ocean bed, although the existence of some of the chief mountain chains has been known (but not in any detail) for many years. But now hundreds of ships of all nations are fitted with an electrical device which makes it possible to record continuously the depth of water over which the ship is moving. The apparatus sends a sound through the water to the bottom of the sea and registers the time it takes for the echo to return. In shallow water the echo is heard after a very short interval; the deeper the water, the longer the time. From all these ships, crossing all the seas of the world, information is accumulating faster than maps can be drawn; but day by day the details are being filled in. A detailed knowledge of the shape of the sea-bed all over the world ______.