When the Fagradalsfjall volcano on Iceland's Reykjanes Peninsula began to erupt in March 2021, ending more than 800 years of dormancy, the event attracted global attention. Thousands of tourists flocked to the site, some venturing within 500 feet of the eruption to witness brilliant red lava flowing from the mountain. Cameras placed around the volcano captured the spectacle in unprecedented detail. Among those watching closely was Francesco Sauro, a 39-year-old Italian geologist and full-time speleologist who serves as president of the geographical exploration society La Venta and collaborates with NASA and the European Space Agency to train astronauts in planetary exploration. From his home in northern Italy, Sauro spent hours each day studying photographs and video footage. The steady stream of visual data was giving researchers the ability to track how and where caves were forming within the cooling lava. It also presented a rare opportunity to study the interiors of cavities that had not yet been touched by any living matter.‍​‌‌​​‌‌‌​‌‌‌​‌​‌​‌‌​​‌​‌​‌‌‌​​‌‌​‌‌‌​‌​​‍

The most common caves on Earth are classified as "destructional," formed when rainwater mixes with carbon dioxide in soil and becomes a weak acid that dissolves soft rock such as limestone. Other destructional caves develop inside formations made of harder material such as basalt, through slow water erosion over long periods. "Constructional" caves, by contrast, are created when flowing lava begins to cool, producing a crusty top layer that solidifies into rock. As the molten lava beneath this crust drains away, it leaves behind a hollow passage known as a lava tube. These tubes can range from a small channel barely three feet across to a large chamber exceeding 150 feet in height, and they may form as a single conduit or a series of interconnected passages stacked one on top of another.‍​‌‌​​‌‌‌​‌‌‌​‌​‌​‌‌​​‌​‌​‌‌‌​​‌‌​‌‌‌​‌​​‍

The urgency of entering the caves quickly was a lesson speleologists had absorbed in 1994, when they studied lava tubes after an eruption of Mount Etna in Italy. Nearly a year after that eruption ended, researchers found rare crystals and minerals inside the tubes, where temperatures still reached a hazardous 158 degrees Fahrenheit. When they returned six months later, those minerals had vanished. They were "metastable," maintaining their structure only at elevated temperatures. As the tubes cooled, the minerals disappeared, and with them the chance to examine them closely.‍​‌‌​​‌‌‌​‌‌‌​‌​‌​‌‌​​‌​‌​‌‌‌​​‌‌​‌‌‌​‌​​‍

To prepare for entry, Sauro's team needed to know exactly where caves were forming and which tubes offered the safest access. Gro Pedersen, a geologist at the University of Iceland's Nordic Volcanological Center, was responsible for gathering imagery. She and Birgir Oskarsson from the Icelandic Institute of Natural History surveyed the volcano from an aircraft every two weeks between March and September 2021, supplementing their observations with drone and satellite data. By combining images from different angles, they constructed both a topographic map and a visual map of the lava flow field.‍​‌‌​​‌‌‌​‌‌‌​‌​‌​‌‌​​‌​‌​‌‌‌​​‌‌​‌‌‌​‌​​‍

Sauro and his team, funded by a grant from the National Geographic Society, reached the volcano in September 2021, about a week after the eruption subsided. Using thermal imaging cameras mounted on drones, they identified "skylight points" on the surface, locations that could serve as entrances to newly formed caves. By May 2022, they could approach several cave openings, but internal temperatures still reached 900 degrees Fahrenheit. The team finally entered one of the caves in October 2022, wearing metallurgical suits and breathing from portable tanks of compressed air because the atmosphere inside was both superheated and laden with toxic gases. In one tube, the cave wall was still glowing at nearly 600 degrees Celsius.‍​‌‌​​‌‌‌​‌‌‌​‌​‌​‌‌​​‌​‌​‌‌‌​​‌‌​‌‌‌​‌​​‍

Two research priorities guided the team. First, they aimed to study minerals forming on cave walls and rocky surfaces. Using a scanning electron microscope housed in a tent beside the volcano, they identified samples within half an hour of collection. Among the expected minerals was mirabilite, composed of hydrogen, sodium, and sulfur. But they also discovered novel substances, including wulffite, an emerald-green crystal previously recorded only once, near a Russian volcano. Fabrizio Nestola, a mineralogist at the University of Padua, is certain that some minerals will prove entirely new to science.‍​‌‌​​‌‌‌​‌‌‌​‌​‌​‌‌​​‌​‌​‌‌‌​​‌‌​‌‌‌​‌​​‍

The second priority involved microbial colonisation. Sauro's microbiologist colleagues collected samples from patches of biofilm on rock surfaces. Analysis revealed that different micro-organisms had colonised different parts of the same cave. Martina Cappelletti, a microbiologist at the University of Bologna, reported that soil-associated bacteria were the first colonisers, likely carried into the caves by air currents. These organisms thrive because they derive energy from oxidising inorganic materials such as sulfur, iron, and copper, requiring neither water nor organic matter to survive. As the caves cooled, microbial diversity increased. The findings suggest that such organisms would have the best chance of establishing a foothold in extreme environments, whether on early Earth or on other planets. Because lava tubes were constantly forming in Martian volcanoes, Sauro explains, they could have been rapidly colonised, becoming "a kind of Noah's Ark for Martian life."‍​‌‌​​‌‌‌​‌‌‌​‌​‌​‌‌​​‌​‌​‌‌‌​​‌‌​‌‌‌​‌​​‍