Water levels in nearly one-third of the boreholes investigated in a recent global study are dropping steadily, mainly due to unsustainable pumping rates to irrigate crops or to supply the human population and industrial growth.

People have relied on buried reservoirs of clean, life-giving water for thousands of years. But there are limits to how much can be sucked out of the ground before the wells run dry - or other unpredictable problems emerge.

Now, evidence of significant, long-term declines in groundwater levels across five continents has emerged in a review study by a group of Netherlands-based researchers, heightening concern about the unsustainable rate of the abstraction of water stocks in many countries.

The study, led by hydrogeologist Dr Feifei Cao and colleagues at the International Groundwater Resources Assessment Centre (Igrac), is based on data from roughly 42,000 monitoring wells in 47 countries, collected over the past 20 years.

They caution about the hidden costs when sinking ever deeper wells to chase declining water stocks.

Quite apart from depleting the water needed to supply taps or nourish crops, the proliferation of drilling and pumping is altering the natural flow of underground water in some areas, exposing people to a range of unexpected risks.

One example comes from West Bengal in India, where groundwater is polluted with sewage or by heavy metals from tanneries. Digging and sucking larger volumes of water can also lead...