The ocean covers 71 percent of the Earth's surface and holds over 1.3 billion cubic km of water.

There is only one global ocean, but it is traditionally divided into 5 parts: The Pacific, Atlantic, Indian, Southern, and Arctic Oceans.

According to the National Ocean Service, just 5 percent of Earth's oceans have been explored and charted--especially the ocean below the surface. The rest remains mostly undiscovered and unseen by human eyes.

Covering approximately 63 million square miles and containing more than half of the free water on Earth, the Pacific is by far the largest of the world's ocean basins.

From the Titanic to Christopher Columbus' Santa Maria, the ocean floor is littered with about 3 million shipwrecks. The oldest wrecks include 10,000-year-old canoes.

The oceans not only have waves, tides and surface currents--they also have a constantly moving system of deep-ocean circulation driven by temperature and salinity. It is estimated to take 1,000 years for a "parcel" of water to complete the journey along this global conveyor belt.

The Atlantic Ocean is still growing because of sea-floor spreading from the mid-Atlantic Ridge, while the Pacific Ocean is said to be shrinking because the sea floor is folding under itself or subducting into the mantle.

The Challenger Deep in the Mariana Trench is the deepest known point in Earth's oceans. In 2010 the United States Center for Coastal & Ocean Mapping measured the depth of the Challenger Deep at 10,994 meters (36,070 feet) below sea level with an estimated vertical accuracy of ±40 meters.

The water pressure at the bottom of the trench is a crushing eight tons per square inch or about a thousand times the standard atmospheric pressure at sea level--the equivalent of being crushed under a pile of 100 fully grown elephants.

Despite the immense water pressure and absolute darkness, the trench is home to a diverse array of organisms including the zombie worm, sea cucumber, and dumbo octopus.