Seen from space, Earth is a blue planet with some land on it. Oceans cover about 71 percent of Earth's surface and hold about 97 percent of its water. Their average depth is close to four kilometers, and the deepest point, the Challenger Deep in the western Pacific, lies about eleven kilometers down. That trench takes its name from HMS Challenger, whose voyage of 1872 to 1876 sounded the deep sea, collected thousands of new species and is often counted as the birth of modern oceanography.

A small share of fresh water

Only about 2.5 to 3 percent of Earth's water is fresh, and most of that is out of easy reach. Most of Earth's fresh water is locked in ice sheets and glaciers or stored underground as groundwater. The lakes and rivers that cities draw on hold well under one percent of the total. This is why the way water moves between sea, sky, land and rock matters so much for human life.

The water cycle

The water cycle is powered by the Sun. Heat evaporates water from the oceans, lakes and soil, and plants release more through their leaves, a process called transpiration. As moist air rises and cools, the vapor condenses into clouds and falls as rain or snow. On land, some water runs off into streams, some soaks into the ground, and some is taken up by plants. More water evaporates from the sea than falls back onto it; the difference falls on land and returns to the sea through rivers and groundwater.

The same water is reused again and again, but it lingers in each part of the cycle for very different spans. A water molecule spends on average about nine days in the atmosphere but thousands of years in the ocean. Water deep in some aquifers or ice sheets can stay put for tens of thousands of years.

Why the sea is salty

Rain is nearly fresh, yet seawater is about 3.5 percent dissolved salts. Rivers carry small amounts of dissolved minerals from weathered rock into the sea, and when seawater evaporates, the salts stay behind. Hot springs on the seafloor add more. Over long spans the ocean's saltiness has stayed fairly steady, because salts are also removed, for example by being locked into seafloor sediments. In 1715 Edmond Halley suggested that the rate at which rivers add salt might reveal the age of the oceans; the idea failed because the sea also loses salt, but it was an early attempt to measure deep time.

Rivers in the sea

Surface ocean currents are driven mainly by the wind, and Earth's rotation bends them into great circling loops called gyres. The Gulf Stream carries warm water from the Gulf of Mexico up the American coast and across the Atlantic. Benjamin Franklin, as a colonial deputy postmaster, wanted to know why mail ships sailing west from England took noticeably longer than merchant ships. His cousin Timothy Folger, a Nantucket whaling captain, explained that the whalers knew the current and steered clear of it while the mail captains sailed against it. Franklin had the current charted and printed around 1770.

Beneath the surface runs a slower, deeper circulation. In the North Atlantic and around Antarctica, cold, salty water becomes dense enough to sink, driving a slow circulation through the deep ocean. Water that sinks may not return to the surface for centuries. Because this circulation moves heat around the planet, scientists watch closely how it may change as the world warms.

Tides

Tides are caused mainly by the Moon's gravity, with the Sun adding a smaller pull. Most coasts see two high tides and two low tides a little more than a day apart. The largest tides, called spring tides, happen when the Sun and Moon line up, at new moon and full moon; the name has nothing to do with the season. The smallest, neap tides, come when the Sun and Moon pull at right angles. Coastline shape matters too: in Canada's Bay of Fundy the water can rise and fall by more than fifteen meters.

Water underground

Groundwater fills the tiny spaces between grains of sand and gravel and the cracks in rock. An aquifer is a body of rock or sediment that holds enough groundwater, and lets it flow freely enough, to supply wells and springs. Despite a common image, most aquifers are not underground rivers or lakes; they are more like a soaked sponge. The top of the saturated zone is called the water table. When people pump water out faster than rain refills it, the water table drops, wells must be deepened, and in places such as Mexico City and California's Central Valley the ground itself sinks.

Worked example: the Ogallala Aquifer

Beneath parts of eight states on the American Great Plains lies the Ogallala Aquifer, one of the largest in the world. Much of its water seeped in over many thousands of years, and in the dry southern plains rain refills it very slowly. From the 1950s, powerful pumps and center-pivot sprinklers turned the region into a major grain and cattle producer. In parts of Kansas and Texas the water table has fallen by tens of meters, and some wells have run dry. In terms of this level, farmers are drawing down stored groundwater far faster than the water cycle returns it. Because the aquifer is shared, no single farmer gains by saving water alone. Some Kansas districts now agree on local limits, with neighbors cutting their pumping together, trading a bit of harvest today for water that will still be there for their children.