At most, Lang expected to find trace amounts of hydrogen so far underground. But the deepest water sample contained so much gas that as it surfaced, bubbles formed in the tube, a phenomenon similar to what happens when you crack open a fresh can of soda.

“We were like, holy crap,” Lang said, recalling her own reaction and Brazelton’s. “There was a lot of swearing involved.”

The waters are chock-full of hydrogen, the fuel needed to power abiotic reactions.

The Building Blocks of Building Blocks

More than six months after the expedition, the team is still processing their enormous number of samples — studying the water chemistry, identifying microbes, characterizing the rocks, and more. “People are going to do a whole alphabet soup of elemental analyses on these rocks,” said Andrew McCaig, a geologist at the University of Leeds who co-led the expedition.

Preliminary models hint that temperatures near the bottom of the borehole might even reach 122 degrees Celsius, the currently known limit for life (though some studies suggest that the limit might sit even higher). Lang cautions that the models require confirmation because they’re based on measurements taken when the borehole temperatures were slightly suppressed by the cool waters circulating during drilling. If conditions are confirmed to be this extreme, though, the depth would allow scientists to study life-fueling chemical reactions without the muddling influence of microbes.

This would be a significant step forward for scientists studying life’s watery origins. “On Earth today, it’s really hard to witness abiotic or prebiotic chemistry because life dominates; life is everywhere,” said Laurie Barge, an astrobiologist at NASA’s Jet Propulsion Laboratory who was not part of the expedition.

Early analyses also suggest that the small organic acid formate is present in the borehole water. Formate is one of the simplest compounds that can form abiotically, from reactions between carbon dioxide and hydrogen, and it may mark an initial step toward the first glimmers of life on early Earth.

“It’s the raw material to build the building blocks,” Lang said. Continued abiotic reactions with formate could produce larger organic compounds like amino acids, which can be strung together into molecules essential for life, such as enzymes and other proteins.

But much of the chemical picture remains fuzzy at Atlantis Massif. The formate deep in the borehole may have formed without microbial help, as it has in the shallower subsurface nearby, but more testing is needed to be sure. The water also contains methane, a compound that some scientists think was vital for early metabolisms and one that could be generated abiotically from reactions with hydrogen. But how methane forms at Lost City is another mystery — it’s “complicated and confusing,” Brazelton said.

Identifying abiotic reactions in nature could inform future lab experiments testing prebiotic chemistry, where researchers can tweak the conditions to more closely simulate early Earth or other worlds, Barge explained. “Lost City is a really special place,” she said.

Hunting for Microbes

Even if the deep borehole isn’t devoid of life, the almost unprecedented quantity of recovered rocky cores will help scientists link shifts in water chemistry and rock types to the few microbes that may eke out a living underground. Studying how microbes survive amid scarce subsurface resources — perhaps by eating hydrogen and other abiotically formed compounds — could help sharpen our picture of early life.

Brazelton in particular is on the hunt for the specific enzymes microbes use to turn hydrogen and small organic compounds into energy. “The whole idea here is that you have chemistry going on in rocks, and at some point, that chemistry turns into life,” Brazelton said. Those enzymes might just be the knob that helps researchers rewind the evolutionary clock to decipher how the earliest metabolisms came to be.

Other efforts are focused on incubating samples from the rock and trying to catch deep microbes in action, explained Fengping Wang, the geomicrobiologist leading this work at Shanghai Jiao Tong University. Wang has been studying life in the subsurface for almost two decades, but she and other deep-biosphere researchers have largely searched for microbes hiding in ocean sediments. “We know very little about the rock microbes,” she said. “It’s one of the last questions in the deep biosphere: What’s in the hard rocks?”