When the call to base camp was over, Gala hiked to the edge of the pool with his partner, the British cave diver Phil Short, and they put on their scuba rebreathers, masks, and fins. They’d spent the past two days on a platform suspended above another sump, rebuilding their gear. Many of the parts had been cracked or contaminated on the way down, so the two men took their time, cleaning each piece and cannibalizing components from an extra kit, knowing that they’d soon have no time to spare. The water here was between fifty and sixty degrees — cold enough to chill you within minutes — and Gala had no idea where the pool would lead. It might offer swift passage to the next shaft or lead into an endless, mud-dimmed labyrinth.

The rebreathers were good for four hours underwater, longer in a pinch. They removed carbon dioxide from a diver’s breath by passing it through cannisters of soda lime, then recirculating it back to the mouthpiece with a fresh puff of oxygen. Gala and Short were expert at managing dive time, but in the background another clock was always ticking. The team had arrived in February, three months before the rainy season. It was only mid-March now, but the weather wasn’t always predictable. In 2009, a flash flood had trapped two of Gala’s teammates in these tunnels for five days, unsure if the water would ever recede.

Gala had seen traces of its passage on the way down: old ropes shredded to fibre, phone lines stripped of insulation. When the heavy rain began to fall, it would flood this cave completely, trickling down from all over the mountain, gathering in ever-widening branches, dislodging boulders and carving new tunnels till it poured from the mountain into the Santo Domingo River. “You don’t want to be there when that happens,” Stone said. “There is no rescue, period.” To climb straight back to the surface, without stopping to rig ropes and phone wire, would take them four days. It took three days to get back from the moon.

The Google car has now driven more than half a million miles without causing an accident-about twice as far as the average American driver goes before crashing. Of course, the computer has always had a human driver to take over in tight spots. Left to its own devices, Thrun says, it could go only about fifty thousand miles on freeways without a major mistake. Google calls this the dog-food stage: not quite fit for human consumption. “The risk is too high,” Thrun says. “You would never accept it.” The car has trouble in the rain, for instance, when its lasers bounce off shiny surfaces. (The first drops call forth a small icon of a cloud onscreen and a voice warning that auto-drive will soon disengage.) It can’t tell wet concrete from dry or fresh asphalt from firm. It can’t hear a traffic cop’s whistle or follow hand signals.

And yet, for each of its failings, the car has a corresponding strength. It never gets drowsy or distracted, never wonders who has the right-of-way. It knows every turn, tree, and streetlight ahead in precise, three-dimensional detail. Dolgov was riding through a wooded area one night when the car suddenly slowed to a crawl. “I was thinking, What the hell? It must be a bug,” he told me. “Then we noticed the deer walking along the shoulder.” The car, unlike its riders, could see in the dark. Within a year, Thrun added, it should be safe for a hundred thousand miles.

The search for life on Mars is now in its sixth decade. Forty spacecraft have been sent there, and not one has found a single fossil or living thing. The closer we look, the more hostile the planet seems: parched and frozen in every season, its atmosphere inert and murderously thin, its surface scoured by solar winds. By the time Earth took its first breath three billion years ago, geologists now believe, Mars had been suffocating for a billion years. The air had thinned and rivers evaporated; dust storms swept up and ice caps seized what was left of the water. The Great Desiccation Event, as it’s sometimes called, is even more of a mystery than the Great Oxygenation on Earth. We know only this: one planet lived and the other died. One turned green, the other red.