UNESCO recently released a map (pdf), nearly a decade in the making, on where the world's aquifers lie. New Scientist explains the significance:
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. Each trans-boundary aquifer holds the potential for international conflict - if two countries share an aquifer, pumping in one country will affect its neighbour's water supply.
Detail shot of Africa and the Middle East after the jump:
(Hat tip: Joshua Keating)