About seventy percent of earth's surface is covered by water, the main reason that Earth is often called the "Blue Planet". All told, the oceans, land and atmosphere hold the equivalent of around a third of a billion cubic miles of liquid water.

But most of that isn't readily available for our use. About 96.5 percent is salty ocean, a little more than two percent is locked up in ice, and a very tiny percent is vapor in the air. That leaves just over one percent as water that's readily available for human use. This fresh water, used for drinking, irrigation, or industry, generally comes from lakes and rivers replenished by rain and snow.

Of course, precipitation doesn't always fall when or where it's needed, so we also pump vast quantities of water from aquifers, underground layers of sediment or soil that hold water. It's estimated that these below-ground reservoirs hold more than 1,000 times the amount of water that falls on land as rain or snow each year. More than half of the U.S. population depends on this groundwater as its main source of drinking water.