Planet Ocean

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.

For scale, put Mount Everest (8,849 m) into the Challenger Deep. Its summit would still be about 2 km under the surface. The deepest ocean is farther below sea level than the highest mountain is above it.

Humans first reached the bottom in 1960, when the bathyscaphe Trieste carried two men down for about 20 minutes on the sea floor. Film-maker James Cameron made a solo dive in 2012, and a few crewed and robotic vehicles have followed. Even so, far fewer people have visited the deepest ocean than have been to space.

Sea water is salty. Salinity is the amount of dissolved salt, and the open-ocean average is about 35 grams of salt per kilogram of water (often written 35 g/kg, or "35 PSU" on older practical-salinity scales). Sea water is therefore a little denser than fresh water: about 1,025 kg/m³ at the surface, against 1,000 kg/m³ for fresh water. That small extra density is what the next lessons build on.