Chemicals have been found in water at least 1.5 billion years old, which is now being examined for signs of microscopic organisms surviving from a prehistoric age.
The water from deep underground pockets is coming bores drilled from a mine 2.4km beneath Ontario, Canada, they wrote in the journal Nature.
"This water could be some of the oldest on the planet and may even contain life," the team said in a statement.
British and Canadian researchers found it was rich in dissolved gases like hydrogen and methane that are able to sustain microscopic life not exposed to the sun for billions of years.
The rocks around the water were dated about 2.7 billion years old, "but no one thought the water could be the same age, until now," the team said.
Analysing the water's composition in the lab, the team estimated that it was at least 1.5 billion years old, possibly more.
Before this discovery, the only other water from this age had been found trapped in tiny bubbles in rock, incapable of supporting life.
