Water covers almost three-quarters of the Earth’s surface. So how much water is hidden under the planet’s crust? A 2021 study in the journal Geophysical Research Letters found that more Earth water is trapped underground in soil or pores in rocks (known as groundwater) than in Earth’s ice caps and glaciers.

“There are approximately 43.9 million cubic kilometers in the earth’s crust [10,5 мільйонів кубічних миль] Grant Ferguson, a hydrogeologist at the University of Saskatchewan and lead author of the 2021 study, told LiveScience.

By comparison, Antarctic ice contains about 6.5 million cubic miles (27 million cubic kilometers) of water; approximately 720,000 cubic miles (3 million cubic kilometers) of Greenland; According to a 2021 study, there are 38,000 cubic miles (158,000 cubic kilometers) in glaciers outside Antarctica and Greenland.

Earth’s oceans remain the largest reservoir of water on the planet, accounting for approximately 312 million cubic miles (1.3 billion cubic kilometers), according to a 2021 study. However, the study stated that the world’s largest water reservoir, other than the oceans, is groundwater.

A 2015 study in the journal Nature Geoscience estimated that 5.4 million cubic miles (22.6 million cubic kilometers) of shallow groundwater is water in the upper 1.2 miles (2 kilometers) of the Earth’s crust. In contrast, the 2021 study looked at groundwater in the upper 6.2 miles (10 km) of the Earth’s crust, Ferguson said.

This discrepancy was due to the fact that previous estimates of deep groundwater (below the upper 2.2 miles of the Earth’s crust) focused only on low-porosity crystalline rocks such as granite. The 2021 study included sedimentary rocks that are more porous than crystalline.

Overall, the 2021 study more than doubled the amount of groundwater believed to exist 1.2 to 6.2 miles below the Earth’s surface; from approximately 2 million cubic miles (8.5 million cubic kilometers) to 4.9 million cubic miles (20.3 million cubic kilometers). . This new estimate is as large as the 5.7 million cubic miles (23.6 million cubic kilometers) they calculated for shallow groundwater.

The crust is typically 19 to 31 miles (30 to 50 km) thick, much thicker than the 6.2-mile depth assumed by the 2021 study, Ferguson said. They focused on the upper part of the Earth’s crust because it is relatively brittle and therefore has broken rocks that can hold water. Below about 6.2 miles, the Earth’s crust becomes much less porous and likely contains water, he said.

Shallow groundwater aquifers, mostly fresh, are used for drinking and irrigation purposes. By contrast, deep groundwater is salty and cannot easily circulate or flow to the surface, largely cutting it off from the rest of the planet’s water, Ferguson noted.

But the relative isolation of deep groundwater means that in some places this brine is trapped for extremely long periods of time. This means it could provide valuable insights into Earth’s past, according to the 2021 study.

“We know very little about these waters beyond a few kilometers, making them a frontier zone for science,” Ferguson said.

Additionally, these ancient waters may support microbial ecosystems that are still active today. Such deep biological communities could shed light on how life evolved on our planet and on other worlds that are likely to have water deep beneath their surfaces.

“There’s still a lot to learn about kilometer-deep water on Earth and other planets,” Jennifer McIntosh, a hydrogeochemist at the University of Arizona in Tucson and co-author of the 2021 study, told LiveScience.