Volcanic islands eventually erode and sink back into the sea. The island of Espanola in the Galapagos is 4 million years old and sinking fast. But as it dies, it offers new opportunities for all sorts of animals.
Geographically the world can be divided into five oceans, the Pacific, Atlantic, Indian, Southern and Arctic. These oceans cover around 70 percent of the earth and although we can't measure it exactly, it is estimated to have a volume of 1370 million cubic kilometres.