By Jonathan Amos | BBC News
Water drilled from rock in a North American mine is among the oldest yet found on Earth, say scientists.
Novel dating techniques used by the Canadian and UK team suggest the fluid is at least 1.5 billion years old.
The water was probably once on the surface and then percolated through the ground where it became trapped at a depth of 2.4km.
The discovery, made under Timmins, Ontario, is reported in this weeks edition of the journal Nature.
Although far from any light source, particular types of micro-organism could survive in the water - it has the right chemistry; it is rich in dissolved gases like hydrogen and methane.
The team is currently testing its samples to determine whether any such lifeforms are indeed present.
"There are similar waters in South Africa with almost identical chemistry that are tens of millions of years old, and they contain microbes that have adapted to that environment," explained Prof Chris Ballentine from Manchester University.
"These are microbes that can survive on the energy from the natural water-rock interactions," he told BBC News.
A positive identification had fascinating implications for our understanding of how life evolved on the early Earth and where it could exist underground today on other planets, such as Mars, Prof Ballentine added.
The water was recovered from deeply buried sulphide ores containing zinc and copper.
The researchers collected the water as mineworkers drilled new exploratory holes.
Temperature increases with depth and so the fluid emerged at 40-50C.
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