Science | June 2024

Journey Into the Fiery Depths of Earth’s Youngest Caves

What Iceland’s volcanoes are revealing about early life on our planet

Speleologists in metallurgical “cooling suits” emerge from the extreme heat of a lava tube formed by the eruption in 2021 of Mount Fagradalsfjall.

Robbie Shone

By Yudhijit Bhattacharjee

Photographs by Robbie Shone

Francesco Sauro first explored a cave when he was 4 years old. He was with his dad, a professor of geography, in the Lessini mountains, near the northern Italian village of Bosco Chiesanuova, where his father had grown up. His dad was also an amateur cave explorer, and the trip was a kind of preordained rite of passage. “The only memory I have about those caves is that I cried,” Sauro recalls. “I was very scared because of the darkness.” When Sauro was 12, and visiting the area again with his family, the founder of a local museum told him that a nearby cave held the bones of ancient cave birds. “In that moment, my curiosity overcame my fear,” Sauro says. From that day on, he was hooked.

Drone — Adrien Briod, of the Swiss drone company Flyability, operates a drone equipped with a lidar scanner to minutely map a network of lava tubes in 3D. Robbie Shone

In the nearly three decades since, the 39-year-old geologist has trekked into dozens of caves around the world: on islands in the Atlantic Ocean, inside glacier mills in the Alps, beneath the forest floor of the Amazon rainforest. In 2013, he discovered some of the world’s oldest caves inside the mountain known as Auyán Tepui in Venezuela. All told, he’s surveyed more than 60 miles of these hidden worlds, including several caves that were unknown to humankind. Some were millions of years old. Others formed tens of thousands of years ago. Recently, he explored caves that are even younger: pristine cavities known as lava tubes, forged inside cooling mounds of molten rock during the eruption of the Fagradalsfjall volcano, in southern Iceland, in 2021. For explorers looking to set foot on uncharted territory, few spaces can match the novelty. But beyond that elemental thrill, these infant caves offer an exceedingly rare opportunity to study cavernous worlds almost from their moment of origin.

Cover image of the Smithsonian Magazine June 2024 issue

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This article is a selection from the June 2024 issue of Smithsonian magazine

Lava Feilds — The researchers cross a lava field on the Reykjanes Peninsula to investigate a cave entrance in May 2023, during the second expedition to the site. Robbie Shone

The most common caves on Earth are formed when rainwater mixes with carbon dioxide in the soil and turns into a weak acid, dissolving soft, soluble rock such as limestone below. Similar “destructional” caves are formed inside mountains and rocky formations made of less soluble material such as basalt, when flowing water slowly erodes the stone over long periods of time. “Constructional” caves, by contrast, are forged when flowing lava begins to cool, creating a top, crusty layer that solidifies into rock. As the molten lava beneath the crust flows out, it leaves behind a new cavity—a lava tube. “These caves are built in an instant of geologic time,” Sauro says. Lava tubes can range in size from a small hollow barely three feet in diameter to a large chamber more than 150 feet tall. They can be formed as a single conduit, or as a series of small, interconnected tubes. Some might be “tiered” one on top of another—a stack of caves.

Scientists — In a tent beside the volcano, Martina Cappelletti, far left, and Ana Miller, both microbiologists, with expedition leader Francesco Sauro. The researchers are examining high-resolution scans of bacteria collected from inside a cave. Robbie Shone

Somewhere between 50 to 70 of the planet’s 1,500 or so active volcanoes erupt every year. When Mount Fagradalsfjall began to erupt in March 2021, capping what had been more than 800 years of dormancy, the world looked on with fascination, in part because an eruption elsewhere in Iceland a decade earlier spewed giant clouds of ash into the atmosphere over Europe, impacting air travel. This time there was no such disruption. Instead, tourists from Iceland and around the world swarmed to the site, some getting within 500 or so feet of the eruption, to glimpse the brilliant red and crimson lava gushing from the mountain and cascading down its sides. “It was the first case where we had cameras everywhere around the volcano, and images coming from the thousands of tourists that were going there to see this incredible show,” Sauro says.

Mineral deposits after exposure to weather and UV light. Because some “metastable” minerals may change over time, researchers strove to retrieve samples quickly. Robbie Shone

Sauro, a full-time speleologist and president of a geographical exploration society called La Venta who also works with NASA and the European Space Agency to help train astronauts in planetary exploration, monitored these developments from his home in northern Italy. He spent hours each day looking at photographs and video footage from the site. This rich stream of information was not just giving researchers the ability to track how and where the caves were forming. It also presented a rare chance to study the interiors of caves that hadn’t yet been touched by living matter: to observe the cooling process, the formation of minerals and the early microbial colonization of those environments in unprecedented detail. And because the caves were formed from lava surpassing temperatures of 1,800 degrees Fahrenheit, the environment inside would be completely sterile. “I was thinking: Hey, as soon as the eruption stops, this will become like an incredible laboratory,” Sauro recalls. “This will become a new world.”

Mount Fagradalsfjall is not actually a single mountain but a cluster of small ridges on a plateau on the Reykjanes Peninsula, about 25 miles southwest of Reykjavik. The surrounding area is flat and covered in moss. The eruption began in a valley between the ridges. As it continued over the next few months, Sauro began making plans. He knew it was imperative to access the caves as soon as physically possible.

Mineral Sample — Miller collects a mineral sample from a cave filled with toxic gases. Among the rare minerals found so far is wulffite, recorded only once before, near a Russian volcano. Robbie Shone

That time was of the essence was a lesson that speleologists had learned in 1994, when studying lava tubes formed after Mount Etna erupted in Italy. When they entered the tubes nearly a year after the eruption had stopped, at which point the temperature inside was still a dangerously high 158 degrees, the researchers found rare crystals and minerals. Returning six months later, however, those minerals were gone. They were “metastable”—holding their form only at high temperatures. As the lava tubes cooled, they had disappeared, and so had the opportunity to examine them in detail.

To prepare to enter the new caves in Iceland, Sauro and his team needed a precise understanding of where exactly they were forming and which tubes presented the easiest and safest access. Gro Pedersen, a geologist at the University of Iceland’s Nordic Volcanological Center, was tasked with collecting images. She and Birgir Óskarsson, from the Icelandic Institute of Natural History, surveyed the volcano from an airplane, flying over it once every two weeks or so between March and September 2021. They also collected other images captured by drones and satellite imagery. “Because of the different angles, we were actually able to create a topographic map, in addition to a good visual map of the lava flow field,” Pedersen says.

Bogdan Onac, a mineralogist, uses a thermal imaging camera to map temperatures inside the cave. One cave wall, still glowing, was recorded at nearly 1,100 degrees Fahrenheit. Robbie Shone

Sauro and his colleagues, who had received a grant from the National Geographic Society, finally got close to the volcano in September 2021, about a week after the eruption subsided. Using their maps, the team identified windows, or “skylight points,” on the surface—locations that were potential entrances into newly formed caves. They flew a drone equipped with thermal imaging cameras over the site to map the temperatures of different parts of the volcanic landscape. In May 2022, they were able to approach the entrances of several caves, but thermal cameras indicated that inside temperatures were still reaching 900 degrees. “There was burning air coming out,” Sauro says. “The winds outside were cold. The contrast between the exterior and the interior was crazy.”

Opener — Giovanni Rossi, center, and Tommaso Santagata through a 1,000-foot-long lava tube—among the youngest caves on Earth. Robbie Shone

Sauro and his expedition members finally entered one of the caves that October, wearing metallurgist suits designed to withstand high temperatures and breathing from portable tanks filled with compressed air, because the air inside was too hot to breathe and laden with toxic gases. The walls were still radiating heat like a furnace, and in certain places the floor was nearly 400 degrees. Sauro and two other team members, equipped with thermal imaging cameras to monitor conditions, advanced cautiously, like a line of soldiers, allowing for the person in the middle and the person in the rear to pull back the line leader in case the expedition suddenly turned dangerous. “The air temperature could change from 100 to 200 degrees [Celsius] in just one meter,” Sauro says. In one tube Sauro entered, the cave wall was still glowing, with a temperature of nearly 600 degrees Celsius (1,100 degrees Fahrenheit). “It was one of the most impressive things I saw,” he says. Pedersen visited the caves after they had cooled further. “I know very few places on Earth where you can go into things that you have seen being born,” she says. “That’s kind of amazing.”

Two lines of research interested Sauro and his colleagues. First, they were eager to study the minerals they would find inside the caves—those formed on the cave walls and other rocky surfaces. Second, they hoped to discover when these extreme habitats would be colonized by micro-organisms and discern which microbes would thrive. Learning how such newly formed caves might begin to harbor life could help researchers refine their ideas about how life developed on Earth, and it would also provide guidance about how and where to look for signs of life, current or past, on other planets, such as Mars. “We know that lava tubes were constantly forming in Martian volcanoes,” Sauro explains. “So they could have been quickly colonized, becoming a kind of Noah’s Ark for Martian life—if life ever existed there.”

Detail #1 — Mineral encrustations offer clues about which microbes first colonize caves—usually those, researchers found, that can derive energy from oxidizing inorganic materials such as sulfur, iron and copper. Robbie Shone

Concerned that some minerals could change or disappear over time, the researchers brought a scanning electron microscope to the site to produce high-resolution images of the samples to help them identify them. Rogier Miltenburg, a technician with the biotechnology company Thermo Fisher Scientific, housed the instrument inside a tent next to the volcano, and he ran a generator inside the tent to maintain the vacuum needed for the microscope to function. The conditions were precarious: Once, when it was raining, a river started to form through the tent. “I had the power supply on the floor, and luckily the water sort of diverted around it,” Miltenburg recalls. “Otherwise we would have had a short.”

Detail #2 — Mineral encrustations offer clues about which microbes first colonize caves—usually those, researchers found, that can derive energy from oxidizing inorganic materials such as sulfur, iron and copper. Robbie Shone

The researchers came across a variety of minerals along fissures and grooves on the cave surfaces. “We found this beautiful white stuff. And then we said, ‘Wait a minute, that’s green there, that’s blue there,’” says Bogdan Onac, a mineralogist at the University of South Florida who was part of the team. Using sterile spatulas, the researchers scraped off samples and packed them in vacuum-sealed bags. Since the temperatures in the lava tubes were so high at the outset, Onac was expecting the minerals to be completely dehydrated crystals, so he was surprised to find some whose texture resembled that of wet sugar, indicating that, in spite of the high heat, water molecules in the environment had been incorporated during mineralization. After collecting samples, Sauro and his colleagues would turn around and walk to the tent for a look at what they had found. By ascertaining a sample’s chemical composition from the images produced by the electron microscope, they could usually identify the mineral within half an hour.

Cover — Rare forms of minerals—including sodium, potassium and copper—grow along a fracture in the walls of a 122 degree Fahrenheit lava tube on the Fagradalsfjall lava field. Robbie Shone

The team had expected to find some minerals such as mirabilite, which is made up of hydrogen, sodium and sulfur. But they also found novel minerals formed from the combination of copper with sodium, potassium, sulfur and other elements, resulting in rare substances that the team is currently studying in greater detail. One surprise mineral, for instance, was wulffite—an emerald-green crystal whose composition includes sodium and potassium along with copper sulfate. “It has only been found once before in the history of mineralogy, in a Russian volcano site,” says Fabrizio Nestola, a mineralogist at the University of Padua. Nestola, who is conducting detailed analyses of the mineral samples at his Padua lab, is certain that some of the minerals will turn out to be entirely new to science, potentially revealing as yet unknown processes by which mineralization takes place.

Sauro’s microbiologist colleagues, meanwhile, collected samples from patches of rock surfaces marked by “biofilms”—areas that had begun to be colonized by bacteria. After extracting samples and analyzing DNA from them at laboratories off-site, the researchers found that different micro-organisms had flourished in different parts of the same cave. “The first data indicate that environmental bacteria, mostly those associated with soil, begin the colonization,” says Martina Cappelletti of the University of Bologna, a microbiologist. “They are probably initially transported inside the cave through air currents.” These micro-organisms can thrive because they are able to subsist on rocks—that is, to derive energy from oxidizing inorganic materials. Over time, as the caves cooled, the diversity of microbes inside the caves increased. The findings suggest that such life-forms, which would not require water or organic matter to survive, should have the best chance to establish a foothold in extreme environments—whether in the distant past or on other planets.

Researcher — Onac inside the microscope tent. Already the researchers have found several rare minerals, he said. And not only that. “Some of them will be new to science.” Robbie Shone

Indeed, tracking microbial colonization will help scientists searching for life elsewhere in the universe. Even on planets where surface conditions today seem inhospitable, lava tubes may once have provided temporary or enduring refuge to life-forms that rapidly colonized the interiors and survived. “If some specific microbial life is able to quickly colonize lava tubes on Earth, why could this not have happened on Mars?” Sauro says.

Collapsed Lava Tube — The view from inside a lava tube whose walls have collapsed. “If you’re there while there are earthquakes—that’s not good,” Sauro deadpanned. Robbie Shone

Penelope Boston, director of NASA’s Astrobiology Institute at NASA Ames, Moffett Field, describes lava tubes as “a model for what we may potentially find on other bodies in the solar system.” And volcanic activity isn’t limited to Earth and Mars. Even Io, one of Jupiter’s moons, has active volcanoes, suggesting that planets and moons beyond our solar system may have volcanoes—and lava tubes—too. That’s why Boston sees great value in studying the caves Sauro is investigating. “I think that designating places around the world where we have this ability to see an early history of microbial colonization from the get-go is something that deserves worldwide attention,” she says.

Lava lake — A small lava lake inside a cave, now solidified. Robbie Shone

Swirly — A swirly segment of a surface lava field, near the volcano crater. Robbie Shone

Wall Detail — A wall detail near a cave entrance. Robbie Shone

The eruption of Fagradalsfjall has subsided, but Sauro has been following news about other volcanoes in Iceland with interest. This past March, when a new eruption started on the Reykjanes Peninsula, at Mount Hagafell, a few miles west of Fagradalsfjall, he mused about “new tubes forming, literally, right now.” These uncharted caverns could be his next hunting ground.