Trapped 1 1/2 miles underground and isolated for untold time, the water contains high levels of methane and hydrogen that could sustain microbial life, they said.
While micrometer-scale pockets in minerals billions of years old can hold water that was trapped during the minerals' formation, no source of free-flowing water passing through interconnected cracks or pores in Earth's crust has previously been shown to have stayed isolated for more than tens of millions of years, researchers at the University of Manchester in Britain said.
But the researchers determined the water captured from fractures in 2.7-billion-year-old sulfide deposits in a copper and zinc mine near Timmins, Ontario, couldn't have contacted Earth's atmosphere for at least 1 billion years, and possibly for as long as 2.64 billion years, Nature reported Wednesday.
"We were expecting these fluids to be possibly tens, perhaps even hundreds of millions of years of age," Manchester geochemist Chris Ballentine said. In collecting the water the researchers took pains to ensure it did not come in contact with mine air, he said.
The isolated water supply, Ballentine said, provides "secluded biomes, ecosystems, in which life, you can speculate, might have even originated."
Ballentine and his colleagues said they are now working to establish whether the water does harbor life.