Huge quantities of nitrate chemicals from farm fertilisers are polluting the rocks beneath our feet, a study says.
Researchers at the British Geological Survey say it could have severe global-scale consequences for rivers, water supplies, human health and the economy.
They say the nitrate will be released from the rocks into rivers via springs.
That will cause toxic algal blooms and fish deaths, and will cost industry and consumers billions of pounds a year in extra water treatment.
In a paper in Nature Communications, the scientists from BGS and Lancaster University estimate that up to 180 million tonnes of nitrate are stored in rocks worldwide - perhaps twice the amount stored in soils.
They say this is the first global estimate of the amount of nitrate trapped between the soil layer and the water-bearing aquifers below. They warn that over time the nitrate will inevitably slowly seep into the aquifers.
Most nitrate, the team says, is in rocks in North America, China and Europe where fertiliser has been lavishly applied for decades.
In some developed countries, the amount of nitrate stored in the rocks is increasing, despite improvements in farming practice and the introduction of rules to control the pollutant.
In developing countries, the problem is currently not so severe. But there is an urgent need for early intervention to avoid the environmental damage experienced by rich countries.