The 1.5 billion-year-old pool of water. Barbara Sherwood Loller/University of Toronto

Deep within a mine in Canada, there is a pool of water bubbling out of the ground. It’s close to 2 miles below the surface of the earth and, according to the scientists who discovered it, it’s been there for 2 billion years, making it the oldest pool of water in the world.

Previously that record was held by a pool further up in the mine, about 1.5 miles down, which was discovered in 2013 and given the age of 1.5 billion years.

The scientists date the water by analysing the gases trapped inside. As the CBC explains, gases like helium and xenon accumulate in the water while it’s stuck in rock fractures. Measuring those concentrations can tell the researchers how old the water is.

What’s unique about this water is that it’s been conserved for all that time. Much of the water on this planet has an even older origin: half of the water on Earth is actually melted interstellar ice that predates the sun.

As the BBC reports, the most fascinating aspect of these billion-year-old pools of water is the possibility that they could reveal more about life on Earth billions of years ago. The scientists have detected signs that single-celled organisms once lived in this water, which is now about eight times saltier than seawater.