They are one of the world's greatest and most precious natural resources, yet are entirely hidden. Now, for the first time, a high-resolution map shows where underground aquifers store vast amounts of water.
The map of "blue gold" (pdf format, 4 MB) is the result of nearly a decade of sometimes difficult talks between neighbouring governments, mediated by UNESCO. The hope is that it will help pave the way to an international law to govern how water is shared around the world.
Aquifers are underground layers of rocks or sediments from which water can be extracted – normally by drilling boreholes or digging wells. They hold 100 times the volume of freshwater that flows down rivers and streams around the world at any time.
What the UNESCO map reveals is just how many aquifers cross international borders. So far, the organisation has identified 273 trans-boundary aquifers: 68 in the Americas, 38 in Africa, 155 in Eastern and Western Europe and 12 in Asia.