Deep in a Canadian mine, already “the world’s deepest base metal mine below sea level,” scientists have found pools of water that date back not millions but billions of years. 

A team of geoscientists from the University of Toronto have been exploring this mine in search of water that’s been conserved longer than any other on earth. In 2013, they reported, in the journal Nature, that they had found a pool of water that they dated to 1.5 billion years ago. In 2016, they announced they had found an even older pool, dating back 2 billion years.

These pools are deep in the earth, about 1.5 and 2 miles, respectively, beneath the surface. The scientists dated them by measuring concentrations of gases including argon, helium, and xenon, which are absorbed by the water as it ages in rock fissures.

Part of 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.) But its age isn’t of interest just because it’s a record: Water this old could have traces of ancient organisms that could give scientists clues about how life on earth worked all those years ago.