Fortunately for Lloyd and her colleagues, their research expedition to collect samples of subsurface life from the subduction zone around the volcano passed without incident.

But just 54 days later, the sputtering burst into a furious torrent of ejected boulders and the spectacular view disappeared, engulfed in a dark, mushrooming cloud of volcanic ash as Poás erupted in what was Costa Rica’s most serious volcanic episode in more than 60 years.

For Lloyd, known for her groundbreaking work in microbial geochemistry, that moment eight years ago was more than a brush with danger — it was a reminder of the high-stakes environments her field depends upon to gain understanding of the deep workings of our planet.

Over the past two decades, that understanding has undergone a radical shift. Lloyd, Wrigley Chair in Environmental Studies and professor of Earth sciences, is part of a global group of scientists behind a major discovery that is transforming how we think about life on Earth. The scientists revealed the existence of a vast, previously hidden world of living biomass inside the planet’s crust. This biosphere consists of active microorganisms, hundreds of millions of years old, that are thriving in extreme environments once thought uninhabitable.

As Lloyd details in her new book, Intraterrestrials: Discovering the Strangest Life on Earth, these subsurface microbes can survive in boiling water, acid — even bleach. Some can “breathe” rocks, metals or electrons. Some are hundreds of thousands of years old. All live in ways completely alien to us surface dwellers.

“Overcoming the challenges of sample contamination to find conclusive proof took years of painstaking work, but the evidence is now overwhelming,” Lloyd says. “These deep-life microbes are real. They’re alive and they have decidedly weird lifestyles — doing strange, fascinating things we’re only just beginning to understand.”

To hunt them down, Lloyd and her colleagues explore permafrost in the Arctic, hot springs in Iceland, rift basins in the American Southwest, and volcanoes in Costa Rica, New Zealand and the Andes. In these places, tectonic activity squeezes ancient groundwater to the surface — much as we might wring out a washcloth — creating natural laboratories for studying Earth’s hidden biosphere.

Each location offers a different window into Earth’s deep history, where materials are exchanged between the surface and the subsurface world. In New Mexico’s rift basins, the ground stretches apart, pulling up ancient fluids. In Idaho, springs near old Yellowstone hotspots mark how the continent has drifted. Iceland, in contrast, sits atop both a spreading center and volcanic hotspot, producing a radically different subsurface chemistry.

Subsurface Survivors

Lloyd — a marine biologist by training whose work builds on the trailblazing research of USC Dornsife professors Kenneth Nealson and the late Katrina Edwards and Jan Amend — also explores microbial life thriving beneath the ocean floor. This research into deep sea sediments enables Lloyd to dive not just into the ocean, but into time itself, peering back to the dawn of life on Earth.

Her team uses drills, push cores and even submersibles to retrieve mud from the ocean bottom, extracting it in cores and slicing it into layers to analyze the microbial life within.

Deep-ocean mud might not sound too thrilling an environment, but to Lloyd it’s a portal into a mysterious and exciting secret world.

Imagine, she says, being a microbe. You fall to the seafloor in a slow rain of organic detritus — dead plankton, river runoff, sunken particles. Then you’re buried, grain by grain. You end up meters below the sea, where nothing much happens. For hundreds of thousands — sometimes hundreds of millions — of years.

“And you keep living for all that time,” she says. “These are not lively lives. There’s not enough energy down there to reproduce. But they persist — barely — metabolizing just enough to survive.”

The implication is staggering — and not just because some of these single-celled organisms may have been alive for a hundred million years. Despite their similar appearances under a microscope, the genetic differences of these organisms are far more profound than any between humans and other visible forms of life on Earth.

They’ve been down there changing slowly, unnoticed for nearly as long as Earth has had life.”

The Life Beneath

On the surface, life displays dazzling diversity — from towering sequoias to luminous jellyfish, from scuttling ants to graceful giraffes. But even the most alien-looking organisms we can see are remarkably close cousins, evolutionarily speaking. “I look at my children and then at a jellyfish — clearly different species,” Lloyd says. “But when we look at their DNA, we find that we only diverged from jellyfish a few hundred million years ago.”

That may sound like a long time ago — but in evolutionary terms, it’s a mere blink of an eye, particularly when compared to the timescale of subsurface microbes. They diverged from one another billions of years ago.

“There are 100 billion billion billion living microbial cells underlying all the world’s oceans,” Lloyd says. “That’s 200 times more than the total biomass of humans on this planet. And those microbes have a fundamentally different relationship with time and energy than we do.”

Another striking way that subsurface life forms differ from those on the surface of the planet — and from each other — lies in what they breathe.

“Most visible life on Earth either respires oxygen, ferments, or photosynthesizes — and that’s it,” says Lloyd. “But some species of subsurface life can ‘breathe’ every metal on the periodic table, including arsenic, while others breathe carbon dioxide. That’s not something any visible life form can do.”

To “breathe,” in this context, means to harvest energy through redox reactions — chemical exchanges through which electrons are transferred. When those reactions are coupled to energy production inside the cell, it’s called respiration. For microbes buried deep underground, oxygen often isn’t even part of the equation.

Even more fascinating, metal-breathing microbes are not confined to one lineage. Just as diverse organisms on the surface have evolved to breathe oxygen, metal breathers appear on wildly different branches of the tree of life — a testament to life’s adaptability and the evolutionary forces shaping it far beneath our feet.

“It makes perfect sense when you think about it,” Lloyd says. “They’ve had 10 times more time to evolve in different directions. They’ve been down there, changing slowly, unnoticed, for nearly as long as Earth has had life.

“Now, the cutting-edge methods we are developing to study the subsurface have enabled us to discover that there are branches on the tree of life that we never knew existed until now — lineages of life that had gone undetected for billions of years,” she says.

This realization reshapes our understanding of biology. It suggests that we’ve only scratched the surface — literally — of life on Earth.

Practical Potential

This extraordinary endurance isn’t just biologically fascinating, it may have real-world applications. One of Lloyd’s collaborators at USC Dornsife, Andrew Steen, associate professor of biological sciences and Earth sciences, is investigating whether protein-stabilizing microbes could help extend the shelf life of vaccines without refrigeration — a breakthrough that could revolutionize access to healthcare in parts of the world with limited resources.

Lloyd’s trip to Costa Rica — which sits on a subduction zone — also helped solve a puzzle with implications for the climate: Why does carbon dioxide escape in massive plumes from the country’s volcanoes, but only in tiny puffs from its hot springs?

Her team discovered that deep underground, microbes and rock reactions were converting the gas into carbonate minerals, locking it into rock before it reached the surface. That insight could inform carbon capture strategies. Instead of releasing CO₂ into the atmosphere, we could pipe it underground, where microbes would help turn it to stone. In some cases, with hydrogen present, microbes can even convert CO₂ into methane — creating a usable energy source, although with climate trade-offs.

From vaccine preservation and carbon capture to enzyme stabilization and insights into aging, Lloyd’s research has far-reaching potential. But she’s quick to point out that real-world benefits aren’t her primary focus.

“I’m not looking for any particular solution or benefit,” she says. “My goal is to push the boundaries of what we know to best provide for other people who are doing practical stuff. That’s the promise of blue-sky research.”

Still, her excitement about the possibilities is palpable. “If we can make such a big breakthrough from just one study, then imagine what else is waiting to be discovered down there?” Lloyd says. “This new field of geobiochemistry is going to have huge implications — maybe even predicting earthquakes or finding life beyond our planet.”

It could even, she says, help us understand the origin of life itself.

“Life buried deep within the Earth’s crust may seem irrelevant to our daily lives,” she says. “But this weird, slow life may hold the answers to some of our planet’s greatest mysteries — and challenges.”