In 2016 the oldest water in the world was found 3 kilometers (1.8 miles) deep at the bottom of a Canadian mine. Since the previous record had been set three years earlier at a higher level of the same mine, it seemed like there might be something special about that location. Now, however, the same team have found water at similar depth in the Moab Khotsong gold and uranium mine in South Africa and it's at least 1.2 billion years old. Like the Canadian water, it contains elements that allow life to survive without any access to energy from the Sun.

Many life forms survive without direct sunlight, for example in caves and at the bottom of the ocean. For most, however, the Sun is still the ultimate source of energy, for example, benthic species rely on foods that filter down from the ocean's surface. Exceptions include life forms living around hydrothermal vents on the sea floor, and microbes that live off hydrogen deep underground.

We've yet to establish the depth limits for such hydrogen-fed life forms, but a new paper in Nature Communications provides evidence exceptionally deep and ancient habitat sites may be quite abundant. The waters beneath Moab Khotsong have higher concentrations of elements produced by radioactive decay than have ever been seen before, and some of them offer opportunities for life.

Dr Oliver Warr of the University of Toronto and co-authors found water within precambrian crystalline rocks 2.9 kilometers below the surface. They note these rocks cover an estimated 72 percent of Earth's continental crust by surface area, and may account for as much as 30 percent of the planet's groundwater.

Reactions between the water and certain rock types produces hydrogen gas here. Although production is slow for any specific interface, over such a large area this can produce an immense volume of gas over time, providing a major energy source for microbes, or perhaps for humans if we can tap it. Some of the hydrogen reacts with carbon to produce methane and more complex hydrocarbons, expanding the range of microorganisms that can be supported.

Meanwhile radioactive decay of unstable isotopes produces alpha particles, which become helium through electron capture, providing a source for arguably the most finite resource. Uranium, thorium and potassium in the surrounding rocks decay to produce lighter elements, including noble gasses (helium, neon and argon) whose concentration builds up over time and provides a measure of the age of the water in which they are trapped.

"Think of it as a Pandora's Box of helium-and-hydrogen-producing power, one that we can learn how to harness for the benefit of the deep biosphere on a global scale." Warr said in a statement. 

The paper does not explore the extent to which life has taken advantage of what Moab Khotsong has to offer. However, future studies may not only reveal a deeply alien ecosystem, they could provide insight into the potential for life deep within other worlds, where water is abundant but sunlight unavailable.

This is only the second example of groundwater aged more than a billion years, but there is one important difference from the previous one. At Canada's Kidd Creek the groundwater's isolation was total, whereas at Moab Khotsong the water could not mix, but lighter noble gasses have escaped by diffusing through the rocks, leading to discrepancies in the concentration between different elements.