So much water has been pumped from beneath Mexico City that buildings have in some places sunk two metres. The water table under the high plains in the American Midwest has fallen on average by three metres a decade and up to 30 metres in some places. So much has been extracted from southern Florida that the aquifers are at risk of flooding by sea water.

Paradoxically, some cities in the Arabian Gulf have become waterlogged because of leaking pipes from coastal desalination plants.

Twelve cities of more than 10 million people - including Bangkok, Shanghai, London and Calcutta - rely on underground water reserves.

"Some two billion people and as much as 40% of agriculture is at least partly reliant on these hidden stores," said Klaus Toepfer, executive director of Unep. "Groundwater also supplements river flows, springs and wetlands vital for rural and urban communities and wildlife. Most of the world's liquid freshwaters are found not in rivers and lakes, but below ground."

The world population has more than doubled in the past 60 years; water is used heavily by both industry and agriculture.

The report, published to mark World Environment Day today, warns that in rural India, 50% of irrigation water and 80 % of drinking water comes from underground, through three million hand-pumped wells.

Around 96% of Saudi Arabia's water and 69% of Bangladesh's comes from below ground.

There are 1,300 boreholes tapping water below Dhaka, the Bangladeshi capital, and in some areas the water table has fallen by 40 metres.

Some 450m people in 29 countries live with chronic water shortages. One person in six cannot rely on safe drinking water.

More than 2bn people have no adequate sanitation. Water-borne diseases kill a child every eight seconds, and are behind 80% of all illness and death in the developing world.

Most of all, water means food. It takes 1,000 tonnes of water to grow a tonne of wheat; 2000 tonnes to produce a tonne of rice. The worry is that the people most at risk - small farmers in poor rural districts - will be the first to suffer as wells dry and water tables sink.

"We need to learn how to value water," said Kofi Annan, UN secretary general. "While in some instances, that may mean making users pay a realistic price, it must never mean depriving already marginalised people of this resource. It is one of the crueller ironies of today's world water situation that those with the lowest income generally pay the most for their water."

Underground water once fed the fountains in Trafalgar Square: for decades, the fountains have been provided by pumps. But ironically, as heavy industry has closed down, water levels below London have begun to rise. Other nations face a future with that sinking feeling.

"Some of the most spectacular Spanish buildings in Mexico City have suffered from subsidence," said John Chilton of the British Geological Survey, one of the report's co-authors.

"Some of the oases in central Jordan are threatened because extraction of groundwater far up the system is diminishing ground water discharge to oases and wetlands."