There's something alive down there. Or at least that's how it looks – much like missing snacks and an emptied beer fridge can make it look like there's an invisible brother-in-law living in your basement.
In this case, the basement is 2.4 kilometres deep in a Northern Ontario mine where scientists having been studying the mysterious reservoirs of ancient water that occasionally come bubbling up out of boreholes in the rock.
In 2013, researchers first revealed evidence that the water has been cut off from the surface for as long as 2.7 billion years. Now, they've taken the next crucial step, showing that the water comes with its own self-sustaining life-support system, like a space capsule that has been out of contact with Earth for eons but is still perfectly livable.
Even more tantalizing, the team reports indirect evidence that the water is inhabited by some form of life that has yet to be identified. The life would be microbial in nature, but potentially separated from life on the surface for so long that it's practically alien, persisting in the depths of Earth's crust with neither sunlight nor atmospheric oxygen to rely on.
The find conjures up the possibility that there are microbial communities evolving in parallel but separate domains scattered around the globe, all hidden far below the surface. And because the same sort of deep watery pockets may be present on Mars, those microbes are analogs for what may turn up on the red planet one day.
"This continues to open up our idea of how much of this planet is habitable," said Barbara Sherwood Lollar, a University of Toronto geochemist and the senior researcher on the team. "And it speaks to the habitability of Mars as well."
Microbes have already been spotted in similar underground environments in South Africa, but in Canada, the water has potentially been isolated for 10 times longer, a separation that spans half the planet's history.
In the team's latest work, published Thursday in the journal Nature Communications, Long Li, now an assistant professor at the University of Alberta, collected samples of the ancient water from boreholes at the Kidd mine, near Timmins, Ont. The water flows there in abundance and is lukewarm, Dr. Li said, sometimes splashing researchers in their faces as they work.
"it's not very pleasant," he added, describing the water's flavour as a combination of salty, bitter and metallic. But what interested Dr. Li most was the water's sulphur content.
Virtually all life forms derive their energy from chemical reactions that shift electrons – electrically charged particles – from one place to another. Humans and other animals do this by marrying electrons derived from the food they eat to the oxygen they breathe.
But some types of bacteria have the ability to exploit alternative energy pathways. One of these involves using hydrogen gas as the electron source and a form of dissolved sulphur known as sulphate as the electron destination. The resulting chain of chemical reactions can create enough energy to sustain a bacterial cell.
Working with colleagues at McGill University, Dr. Li found that the sulphur in the Kidd mine water is supplied by the mineral pyrite in the surrounding rocks. The natural radioactivity of the rock produces enough energy to split a small fraction of the water into hydrogen and oxygen, with the oxygen used to dissolve the sulphur and turn it into sulphate. The chemical signature of the sulphate showed the process has likely been going on since the water was first isolated underground, creating a long-term sustainable environment for bacteria.
In a second part of the study, Dr. Li found that the amount of sulphate was 100 to 1,000 times less than would be expected based on lab experiments. The most likely explanation, he said, is that microbes are using up the available sulphate to survive. The rate of consumption suggests the microbes are present in extremely low numbers, making their direct detection a major challenge. Dr. Sherwood Lollar said the team is working with microbiologists in an effort to spot any trace of the water's suspected inhabitants.
Alex Sessions, a professor of geobiology at the California Institute of Technology said the result represented an important find.
"The thing that's special is that the sulphate is being generated by radioactive decay," added Dr. Sessions, who was not involved in the study. It means that on countless planets, moons or asteroids that lack oceans, volcanoes and deep sea vents – all of which help support ecosystems on Earth – alien life may be surviving on little more than water, sulphur and radioactive elements.
"That's huge," he said.