The ocean’s depth ranges from 600 feet or less above continental shelves to more than 35,000 feet at the Marianas Trench. The mean depth is 12,451 feet. Sea bottom topography includes wide plains, the world’s longest mountain range, steeply rising individual truncated peaks called _guyots_ (pronounced gee-ohs), gentle slopes, narrow canyons, and precipitous escarpments. Mountains higher than Everest rise from the ocean floor and never pierce the surface. [Illustration: _Underwater mountain traced by the Woods Hole Oceanographic Institution echo sounder in the Caribbean area. Depth is determined by the time it takes the sound emitted by the instrument to go to the bottom and return to the surface._] Ocean Movements [Illustration: _Six ships checking the Gulf Stream’s course through the Atlantic Ocean over a 2-week period found the variations shown above._] [Illustration: _The infrared film photograph shows the edge of the Gulf Stream. The visible line between the Gulf Stream, which is on the right, and Labrador water is made by Sargassum weed concentrated at the interface._] The ocean is constantly in motion—not just in the waves and tides that characterize its surface but in great currents that swirl between continents, moving (among other things) great quantities of heat from one part of the world to another. Beneath these surface currents are others, deeply hidden, that flow as often as not in an entirely different direction from the surface course. These enormous “rivers”—quite unconstant, sometimes shifting, often branching and eddying in a manner that defies explanation and prediction—occasionally create disastrous results. One example is El Niño, the periodic catastrophe that plagues the west coast of South America. This coast normally is caressed by the cold, rich Humboldt Current. Usually the Humboldt hugs the shore and extends 200 to 300 miles out to sea. It is rich in life. It fosters the largest commercial fishery in the world and is the home of one of the mightiest game fish on record, the black marlin. The droppings of marine birds that feed from its waters are responsible for the fertilizer (guano) exports that undergird the Chilean, Peruvian, and Ecuadorian economies. Every few years, however, the Humboldt disappears. It moves out from shore or simply sinks, and a flow of warm, exhausted surface water known as El Niño takes its place. Simultaneously, torrential rains assault the coast. Fishes and birds die by the millions. Commercial fisheries are closed. The beaches reek with death. El Niño is a stark demonstration of man’s dependence on the sea and why he must learn more about it.