Sketch of the Marianas Archipelago showing the Marianas Trench
"Grand Canyon of the Pacific", The Marianas Trench is the world's greatest ocean depth. The island of Saipan rises more than 36,000 feet above the floor of the trench. On the floor of the trench the water pressure is more than 7 tons per square inch. In the Pacific there is always an association between the arc of a chain of islands and deep ocean trenches. The southern islands are composed predominantly of madrepore limestone while the northern islands are composed of volcanic rock. The islands rise out of the sea in successive level terraces indicative of repeated volcanic elevations. Farallon De Medinilla is a coral island with a basalt nucleus. Several volcanoes in the northern islands are active. While earthquakes occur in the Marianas, the vast majority are registered only as seismograph and not felt by the inhabitants.