The total amount of groundwater on the planet

Water held in rock and soil below our feet, is estimated to be 23 million cubic km. The calculation comes from a Canadian-led team and is published in the journal Nature Geoscience. Significantly, little of this water - just 6% - is the kind of bankable resource that is most useful to people. Significantly, little of this water - just 6% - is the kind of bankable resource that is most useful to people. That small fraction is referred to as "modern" groundwater.

Water held in rock and soil below our feet, is estimated to be 23 million cubic km. The calculation comes from a Canadian-led team and is published in the journal Nature Geoscience.

Significantly, little of this water - just 6% - is the kind of bankable resource that is most useful to people. That small fraction is referred to as "modern" groundwater: it is extractable because it is near the surface, and can be used to supplement above-ground resources in rivers and lakes.

This modern groundwater is also the most sensitive to climate change and to human contamination.

To quantify just how much water is stored in the top 2km of the Earth's surface, the researchers combined large data sets with an element of modelling.

They included information on the permeability of rocks and soil, on their porosity, and all that is known about water table gradients, which tell you about inputs from precipitation.Key to determining the age of all this stored water is a collection of thousands of tritium measurements.

Tritium is a radioactive form of hydrogen that spiked in the atmosphere 50 years ago as a result of thermonuclear bomb tests. It can therefore be used as a tracer for all the rain that has made its way underground ever since.

The study underlines just how unevenly modern water reserves are spread around the globe and the urgent need to raise awareness that our youngest groundwater resources - those that are the most sensitive to anthropogenic and natural environmental changes - are finite.