Oceans cover over 70% of Earth's surface and, across the globe, have an average depth of about 12,500 feet. Of course, this average reflects a wide range of underwater topography, from shallow continental shelves (averaging less than 500 feet deep) to ocean trenches (often more than 25,000 feet deep).
The Southern Ocean, though the fourth largest, actually has the greatest average depth (near 14,500 feet) while the Arctic Ocean, the smallest of our five primary bodies of water, is the shallowest, averaging about 3400 feet in depth. The Pacific Ocean, which covers a third of the globe, has an average depth of 14,000 feet, the Atlantic averages 11,000 feet and the Indian Ocean averages about 12,800 feet. The deepest areas of the sea occur in ocean trenches, where an oceanic plate is subducting beneath another tectonic plate; the Mariana Trench, just east of the Mariana Islands and Guam, plunges to a depth of 36,000 feet, the deepest point on the ocean floor. Other well known trenches (and their approximate maximum depths) include the Philippine Trench (34,600 feet), the Japan Trench (30,000 feet), the Puerto Rico Trench (28,200 feet), the Atacama Trench (26,500 feet), the Java Trench (25,400 feet) and the Aleutian Trench (25,200 feet). The Gulf of Mexico is generally shallow (40% is less than 80 feet deep) but the Sigsbee Deep, in its southwest quadrant, reaches depths of more than 14,000 feet.
Sunlight only penetrates about 400 feet below the surface of the sea. When one considers the extent and depth of our oceans, we come to realize that nearly 70% of Earth's living ecosystem (sea and land combined) lies in perpetual darkness!
