The depths of Earth’s crust hold a huge volume of ancient, salty water that has been undetected until now.

Grant Ferguson at the University of Saskatchewan in Saskatoon, Canada, and his colleagues calculated how much of this underground water should exist. They analysed a global database of the types of rock that make up the uppermost 10 kilometres of the planet’s continental crust. Nearly 88% is hard crystalline rock, and 12% is sedimentary rock, which has large spaces between its grains.

The scientists calculated how much water could exist between the grains of both of these rock types, and estimated that the uppermost 10 kilometres of Earth’s crust holds nearly 44 million cubic kilometres of water. That’s more than the amount frozen in glaciers and the ice sheets of Greenland and Antarctica.

Most of this vast reservoir lies at a depth of between 1 kilometre and 10 kilometres, beyond the reach of wells that could tap it. The groundwater used by many farmers for irrigation and by billions of people for drinking is at much shallower depths.