But if life did arise on Mars, might it have migrated to the underworld and persisted?

For a couple of decades, Dr. Onstott has been talking his way into South African gold mines, regaling the mine managers with the wonder of deep Earth life to overcome their wariness. In many ways, the mines provide easy access to the depths — a ride in a cagelike elevator, jammed against miners starting their shift, descending quickly as lights from the different levels zip past. Think of it as traveling through a 450-story skyscraper, going down.

Dr. Onstott and his colleagues had made repeated pilgrimages to this particular tunnel in this particular mine, Beatrix, 160 miles southwest of Johannesburg.

When miners carve out new tunnels, they poke holes through the rock to see what surprises might lie ahead. Sometimes the borehole taps into a section of fractured rock with water coursing through. Then the fracture is drained and plugged.

Image A new species of nematode that scientists discovered in the Beatrix mine. Credit... Gaetan Borgonie/University Ghent, Belgium

But this particular tunnel at Beatrix never entered production, so the borehole valve remains, allowing the scientists to return to draw samples from the same place.

At this level, almost a mile underground, the elevator gates open to a well-lit, concrete cavern with the unremarkable plainness of a parking garage. A minirailway system transports miners and ore back and forth. The side tunnel, though, is pitch black save for the helmet lamps, and the trek to the valve is a slosh through muck and over tangles of mangled electrical cabling.

Scientists led by Dr. Onstott made their most recent trip to South Africa in June last year. Over a couple of hours, they took their fill of the water and set up an apparatus that remains attached to the valve, trapping microbes, which were retrieved later in the summer. Since then, they have been analyzing the samples to understand this assemblage of life.