World Oceans and Seas

Earth's oceans cover approximately 70.8 percent of the planet's surface, an area of roughly 361 million square kilometers. The ocean contains about 97 percent of all water on Earth, with a total volume estimated at approximately 1.335 billion cubic kilometers. Because water has a high heat capacity and circulates across the entire globe, it moderates temperatures, distributes moisture, and shapes weather patterns on every continent.

Geographers, oceanographers, and international bodies generally describe the ocean as a single body of salt water known as the World Ocean. For practical reference, this continuous expanse is subdivided into five major basins: the Pacific, the Atlantic, the Indian, the Southern, and the Arctic oceans. These boundaries are conventions rather than physical barriers. Currents, nutrients, heat, and marine species move between the basins continuously, and the behavior of one ocean is inseparable from the behavior of the others.

The ocean has an average depth of approximately 3,688 meters. Its deepest known point is the Challenger Deep, located in the Mariana Trench in the western Pacific, where the seafloor descends to approximately 10,935 meters below the surface. Beyond this single trench, the global seafloor includes abyssal plains that stretch for thousands of kilometers, mid-ocean ridges that form the longest mountain chain on the planet, volcanic seamounts, deep canyons carved by sediment flows, and hydrothermal vent systems that host entirely chemosynthetic ecosystems.

Despite centuries of exploration, the ocean remains the least understood environment on the planet. Satellite gravimetry has produced a low-resolution map of the entire seafloor, yet only a minority of that seafloor has been charted at the high resolution available for most of the land surface. The ocean's vast volume, extreme pressures, cold temperatures, and perpetual darkness below the photic zone make it both a technological frontier and a scientific one.