The oceans cover 70% of the earth's surface. Most of this volume is poorly studied and understood. Looking at the surface, one could well believe that the sea-floor is flat and featureless. If the water could be removed, one would see that sea-floor is every bit as variable as the land surface, with mountains and valleys. In the middle of the Atlantic Ocean, the second largest ocean on earth, extends an enormously long mountain ridge from north to south: the mid-Atlantic Ridge.
Geologically active parts of the ridge actually break the ocean surface as islands, such as in Iceland or the Azores. Such volcanic ridges are found extending through all the world's oceans. With a length of over 50,000 km, they form the earth's longest volcanic chain. Geologically the ocean ridges are created as the tectonic plates move apart; areas called spreading zones. The "spreading" is caused by the movement of magma deep within the earth.