Groundwater is much less obvious than surface water. Most of it occurs not in subterranean rivers and lakes, but just in saturated layers of earth; alluvial sediments and weathered or fractured rock, known as aquifers. About 30% of the world's people get their water from wells, into which the water seeps quite slowly from the surrouding materials.

There is a lot of water down there; over 15 million km3, or about 40 times the amount of surface water in storage! Like the surface water, however, most of this water is not in the right place, or close enough to the surface, to be accessible to those who need it.

About 30 thousand km3 is replenished each year directly by deep percolation from the surface after a rainfall, or by inflow from surrounding streams and rivers passing above or through the aquifer. This is the renewable groundwater supply, and it would be enough for about 5 million litres per day, for everybody, even with 6 billion people on earth, if it were better distributed or more accessible.

A small amount of groundwater flows naturally, from springs or artesian wells, but most of groundwater development over the last 100 years has involved well drilling and pumping. With the majority of the world's aquifers now overdrawn, there is likely to be much less drilling activity in the 21st century.

Groundwater also gets badly polluted these days, and aquifers are often destroyed by seawater intrusion, when well-fields too close to the ocean's edge are continuously overpumped.

There are also 'fossil' aquifers. These are subterranean depressions filled with things like earth, gravel, sand and clay. They became saturated very long ago in a historically rainy period. Then the water just sat there till we came along and, over the last hundred years or so, started to pump it out. Naturally, as it empties, water levels drop and pumping becomes more costly - then the pumping stops. This is a contentious business. Personally, I don't believe it will do anybody much good to leave that water there another 10,000 years, but some people call it 'mining' water, and are very dissaproving.