Scientists have discovered ancient pockets of water - dating back to at least 2.7 billion years - which contain abundant chemicals known to support life, lurking deep underground in Canada.
This water could be some of the oldest on the planet and may even contain life. Also, the similarity between the rocks that trapped it and those on Mars raises the hope that comparable life-sustaining water could lie buried beneath the Red Planet's surface, scientists believe.
Researchers from the universities of Manchester, Lancaster, Toronto and McMaster analysed water pouring out of boreholes from a mine 2.4 kilometres beneath Ontario, Canada.
They found that the water is rich in dissolved gases like hydrogen, methane and different forms - called isotopes - of noble gases such as helium, neon, argon and xenon.
The hydrogen and methane come from the interaction between the rock and water, as well as natural radioactive elements in the rock reacting with the water. These gases could provide energy for microbes that may not have been exposed to the Sun for billions of years.
The crystalline rocks surrounding the water are thought to be around 2.7 billion years old.
Using ground-breaking techniques developed at the University of Manchester, the researchers show that the fluid is at least 1.5 billion years old, but could be significantly older.
"We've found an interconnected fluid system in the deep Canadian crystalline basement that is billions of years old, and capable of supporting life," said Professor Chris Ballentine of the University of Manchester, co-author of the study, and project director.
"Our finding is of huge interest to researchers who want to understand how microbes evolve in isolation, and is central to the whole question of the origin of life, the sustainability of life, and life in extreme environments and on other planets," Ballentine said.
Before this finding, the only water of this age was found trapped in tiny bubbles in rock and is incapable of supporting life. The water found in the Canadian mine pours from the rock at a rate of nearly two litres per minute.
It has similar characteristics to far younger water flowing from a mine 2.8 kilometres below ground in South Africa that was previously found to support microbes.
"Our Canadian colleagues are trying to find out if the water contains life right now. What we can be sure of is that we have identified a way in which planets can create and preserve an environment friendly to microbial life for billions of years," said Dr Greg Holland of Lancaster University, lead author of the study.
"This is regardless of how inhospitable the surface might be, opening up the possibility of similar environments in the subsurface of Mars," Holland said.
The study was published in journal Nature.