Seen from space, Earth is mostly blue. About 71 percent of the surface is ocean, and the oceans hold roughly 97 percent of all the water on the planet. Most of the rest is frozen in ice sheets and glaciers or hidden underground. Lakes, rivers and clouds together are a tiny sliver.

The ocean is also deep. The average depth is about 3,700 m, so a typical patch of sea floor lies more than three kilometres below the waves. The deepest known spot is the Challenger Deep at the bottom of the Mariana Trench in the western Pacific, at about 10,900 m. Surveys disagree by a few tens of metres (published figures run from roughly 10,900 to 10,935 m) because sound speed, tides and instrument calibration all matter at that depth.