Thousands of islands are scattered over the worldˇs oceans. While some are in the process of being born, others are slowly disappearing under the oceanˇs surface. There are several phenomena responsible for forming islands. Coral or debris may build up on the ocean floor. Rising water may completely surround a portion of land. Dropping water levels may make a shallow area appear above the surface. Most oceanic islands, however,.are nothing more than the peaks of volcanic mountains rising above the waterˇs surface. Over time, the volcano dies out, and the island, worn down by the water, disappears and becomes an undersea mountain.