In 2013, scientists were stunned to find microbes thriving deep inside volcanic rocks beneath the seafloor off the Pacific Northwest, buried under more than 870 feet of sediment. The rocks were on the flank of the volcanic rift where they were born, and they were still young and hot enough to drive intense chemical reactions with the seawater, from which the microbes derived their energy.

Now, however, another team of researchers have discovered living cells inside exceedingly old, cold oceanic crust in the middle of the South Pacific. It isn’t yet clear how these new microbes are managing to survive—and yet, there seem to more than a million times more of them, for the same volume of rock, than in the younger crust.

“Honestly speaking, I couldn’t believe it,” said geoscientist Yohey Suzuki of the University of Tokyo, recalling when he first saw thin sections of the ancient rocks teeming with cells. Suzuki is lead author of the new study, published today in Communications Biology.

View Images Fluorescent microscopy image of bacteria (green) in a rock sample the team collected from below the seafloor. Image by Yohey Suzuki, University of Tokyo

The discovery of microbial life in such an unlikely place supports the astounding possibility that it might be present throughout the oceanic crust—a layer of rock that is as thick as Mount Everest is tall in places and extends over three-fifths of the planet’s surface. It also has broader cosmic implications: There are similar volcanic bands on Mars, a planet that once had a waterlogged surface, perhaps even a colossal ocean.

Roughly four billion years ago, Mars’ outer core stopped churning, its magnetic field collapsed, its atmosphere was stripped away by the solar wind, and it became a desert world. But if that water once brimmed with life, and some of it drained into the ground, biology could still exist in the microscopic cracks of Mars’ buried volcanic rocks—much as it does today within Earth’s oceanic crust.

“If there is an ocean, life is coming through those veins,” says María-Paz Zorzano, a senior scientist at Spain’s Center of Astrobiology, who was not involved with the new work.

Life in a global conveyor

Oceanic crust has been made almost continuously for 3.8 billion years at mid-ocean ridges, a network of volcanoes that stretches 40,000 miles around the planet. Mostly composed of a type of rock named basalt, this newly frozen lava is still hot, and mixes vigorously with cold seawater, creating chemical reactions that provide energy for microbial life on the seafloor—and, it’s now clear, far below it.

Close to mid-ocean ridges, hot young rock is packed with various metals, including iron, in chemical states that readily react with the oxygen in seawater. Microbes there take advantage of that quirk of chemistry and make their own energy from it.

On the flanks of those ridges, however, the seawater’s oxygen has been consumed by all that earlier chemistry. The water-basalt reactions produce hydrogen instead, and as Aarhus University ecologist Mark Lever and his colleagues reported in 2013, enterprising microbes hiding within 3.5-million-year-old oceanic crust use this hydrogen to convert carbon dioxide into life-sustaining organic matter.

Travel farther along inside this conveyor belt of crust—with younger rock forged at ridges pushing away older rock as it’s made—and you’ll find aged cold rock and a dearth of key chemical ingredients, so expectations for microbial life here were low. But that didn’t stop scientists from looking.

Back in October 2010, researchers travelled more than 400 miles west of the Cook Islands. In this lonely part of the vast South Pacific, they drilled into the tough oceanic crust 19,000 feet below their ship.

With so few nutrients available above the drill site, “there is hardly any life in the water”, says Lever, who was not involved with the new research. It is easily one of “the deadest parts of the world’s oceans.”

Several cores of crust were extracted from over 330 feet below the seafloor at multiple sites; the youngest was 13.5 million years old and the oldest 104 million years old. During next decade, Suzuki and his team painstakingly studied the rocks and found that in every sample—in many tiny, iron-rich, clay-filled microfractures—there was life.

Critter-filled vaults

To make sure that no microbe-rich seawater contaminated the samples, the team carefully sterilized the outside of the rocks before cracking them open. The life-forms inside looked to be the crust’s genuine inhabitants, Zorzano says.

The fact that a sprawling, hyper-dense community of microbes was found living in these rocks—an environment crushed under 580 atmospheres’ worth of pressure, with meager nutrients and clogged-up voids to inhabit—is a testament to the enterprising nature of microbial life.

Genetic profiles suggest these crustal communities are dominated by bacteria known as heterotrophs. Unlike the hydrogen thieves in younger oceanic crust, these microbes cannot synthesize their own food and instead need to find food in their surrounding environment. In this case, they appear to get their energy from organic matter.

The heterotrophs’ food could come from either the waste and decomposed remains of marine life that snows from the sea above, or from the non-biological chemical breakdown of the crust itself, as is observed at some deep-sea hydrothermal vent sites. Either way, it gets trapped and concentrated in those clay-filled microfractures, making the clay a “magic material” for life, Suzuki says.

Microbes that eat methane were also found in these old basaltic rocks. The source of methane is unclear, Lever says, but it may have formed in fresh ocean crust through the cooking of trapped carbon dioxide. Perhaps, then, these critters are surviving on leftovers that are tens of millions of years old.

Life beyond Earth

The existence of microbial communities in ancient ocean crust also bodes well for the possibility of life on the red planet. Earth’s oceanic basalts are chemically very similar to Mars’s own basalts, says Arya Udry, a planetary scientist at the University of Nevada, Las Vegas who wasn’t involved with the work.

Does this new discovery boost the chance that similar life could be found in comparable places on our planetary neighbor? “Absolutely,” Lever says. Although its origins remain unclear, methane is present on Mars, which means some of those methane-eating, crust-inhabiting microbes found under the South Pacific could exist in some form on the red planet too.

The clay mineral smectite, which helped supply food to many of the terrestrial microbes, is also found in and atop Martian basalt. “If life existed on Mars in the past, it seems like it would also be very likely to exist today in these deep sub-surface environments,” Lever says.

And if microbes are surviving inside Mars, shielded from the deadly radiation on the planet’s surface, we might soon be able to find it, Zorzano says. The ESA-Roscosmos Rosalind Franklin rover, due to blast off for Mars in 2022, will land in a site filled with clay rich in organic molecules to hunt for biosignatures. And NASA’s Perseverance rover, slated to launch this summer, will gather dozens of rock samples from a clay-rich crater as part of a decade-long effort to send pristine samples back to Earth.

The implications of the new study findings go even beyond our solar system. Many of Earth’s ecosystems are built on the foundations of photosynthetic organisms, from algae floating atop the sea to plants on land. But those methane-eating microbes may extract their energy from the oceanic crust alone, making their ecosystem wildly different but no less successful.