ByRebecca Dzombak
Published February 1, 2024
Near the peak of Mount Rainier, tucked within the majestic volcano’s east crater, lies a network of dark and vast ice caves. These caverns—the largest glaciovolcanic caves in the world—sit more than 14,000 feet above sea level. Along with the altitude, invisible pockets of lethal gases and razor-sharp rocks make these caves an inhospitable place to be, let alone to conduct research.
This icy labyrinth in the glaciers atop Mount Rainer, which towers to the southeast of Seattle, Washington, contains clues about the volcano, the inner workings of glaciers, and even icy worlds far from Earth. The caves can be awe-inspiring, but for caver and National Geographic Explorer Christian Stenner, the joy is only felt once he and the rest of the expedition team are safe and sound.
“It was amazing to set foot in it for the first time,” Stenner says. The main passage of the caves is a large ring around the volcanic crater, but entrances to the system are narrow and cramped, branching up and out from the main ring. Explorers must crouch as they make their way down from entrance holes at the glacier’s surface, crawling along the steep crater to the main ring nearly 500 feet below. “It takes a minute to orient yourself to what you’re about to do, and then you just get to work,” Stenner says.
From 2014 to 2017, that work for Stenner and his team meant painstakingly surveying more than two miles of ice caves, expeditions that were supported by National Geographic. This network was mapped in expeditions in the 1970s and 1990s, but there remained gaps, inconsistencies, and uncertainty. Had tunnels been missed during previous expeditions, or were they newly formed?
Now, in a new study in the Journal of Cave and Karst Studies, Stenner and his collaborators have produced the most complete map of Mount Rainer’s ice caves to date, including nearly twice as much cave length as had been charted before. The team also discovered a subglacial lake which may be the highest-elevation lake in the United States.
Remarkably, these glaciovolcanic caves have persisted for decades, while many such caves are more ephemeral. To find out the secrets to the caves’ longevity and study how they might be impacted by the changing climate, scientists had to venture into the frozen mouth of a volcano.
The caves’ long history
As the name implies, glaciovolcanic caves form in glaciers on top of volcanoes, and even geologists consider them oddities. There may only be about 250 volcanic systems in the world capable of hosting glacial caves, and of those, only a handful in places such as Antarctica, Iceland, and western North America have documented caves.
“Most people haven’t heard of glaciovolcanic caves,” says Linda Sobolewski, a geologist at Ruhr University Bochum in Germany who works with Stenner. “It’s so difficult to enter these caves and do research, so there’s a lot more that needs to be done.”
Reaching such caves requires mountaineering, caving, and ice climbing experience, as well as coping with the physical and mental strain of spending multiple days at high altitudes. As a result, the study authors posited, glaciovolcanic caves may be the least well-understood type of cave on Earth.
The ice caves on Mount Rainier were first documented by mountaineers during a summiting effort in 1870, though Indigenous stories about the mountain—which is also called Tahoma or Takoma, meaning “mother of all waters” in the Indigenous Lushootseed language—are much older. In an account of the expedition published in The Atlantic in 1876, the author describes “a deep cavern, extending into and under the ice, and formed by the action of heat … Its roof was a dome of brilliant green ice with long icicles pendent from it.”
Mountaineers would continue to visit the caves, and scientists also visit for insights into volcanic and glacial processes—on this planet and beyond. A dizzying array of extremophile bacteria live in the caves’ depths, offering insights into what life on icy ocean worlds like Saturn’s moon Enceladus might look like and how humans might one day explore those environments. NASA has even tested ice cave-exploring robots in the ice caves of Rainier.
Changes in the caves’ structure could also alert scientists to subtle volcanic activity, such as the migration of vents for hot volcanic gases called fumaroles, that might not be picked up by remote sensing of the surface. With millions living in the mountain’s shadow, Rainier “is a volcano that needs every bit of monitoring we can throw at it,” Stenner says.
And if someone needs to be rescued from the caves, the rescuers will need a map, says Stenner. (Rainier National Park strongly discourages people from entering the caves because they are such hazardous environments.)
But the strongest motivation for Stenner is to find something that has never been seen before.
“It’s true exploration,” he says. “It’s being in parts of the cave that no human has ever been to, and perhaps never will go again. It’s adding our knowledge to the world. That’s really the reward.”
There and back again—to map
After the 1870 ascent, mountaineering reports and a smattering of research papers provided often-conflicting hints about the caves. To resolve this uncertainty, from 1970 to 1973, a team climbed into the caves properly equipped to produce an accurate map. They documented about 5,900 feet of passage. Then between 1997 and 1998, another team went to the caves with newer equipment and mapped them again, this time charting around 4,900 feet.
Stenner and his colleagues suspected they could improve upon that. Between 2014 and 2017, their team led multiple expeditions to the summit. With more than 80 scientists and volunteers, the 2017 expedition was the largest in the volcano’s history.
Everyone who went up faced a roughly 15-mile trek, gaining 9,000 feet of elevation, lugging a 70-pound pack, and facing 80-mile-per-hour winds. Once they got to the top, the work didn’t stop.
“The last couple miles are really demoralizing. You get to 12,000 feet and realize you still have 2,000 more feet to go,” says Lee Florea, a geologist with the Washington Geological Survey who was a caver on the expeditions. “At 14,000 feet, everything is just brutal on your body.”
Florea lost a few toenails to the steep climbs, and he recalls “blisters the size of softballs” covering the feet of a team member as a medic rushed to save what skin he could. “His feet basically sloughed off inside his boot,” Florea says.
Once camp was set, Stenner and the team hauled small survey stations through the caves, carefully measuring distances and triangulating points to create a digital map. When more nuance was needed, the team hand-drew details and corrections, just as their predecessors had.
When they returned in subsequent years, some of the stations had disappeared into crevices or degraded beyond function. Even when the stations were set up, steam rising from the fumaroles sometimes blocked the instruments’ lasers, preventing them from measuring anything.
“The thing about these caves is that they wreck everything,” Stenner says.
Work in the smaller passages along the edge of the cave system was particularly challenging. Some of the map labels, such as “Misery Crawl” and “Murphy’s Law,” hint at the team’s mood.
But there was also beauty to be found. After spending endless minutes dragging themselves along the jagged crater floor, Stenner and a teammate finally popped out of a hole in the ice on the side of the crater. Below them, the cloud tops stretched out, the mountain casting a blue shadow on white.
“We were in such misery from what we had just done, and we looked back and just had this majestic scene,” Stenner recalls. “It was truly amazing.”
Remarkably stable
After the expedition wrapped up, Stenner and his colleagues—safe again at sea level—methodically compared their new map to the 1970s and 1990s maps, checking for passages that had been missed, or that had recently formed, disappeared, or changed size.
Their new map documented 11,788 feet, about 2.2 miles, of passage. The team estimates roughly 2,000 feet of new passage formed recently. While the main passage, which nearly rings the entire crater, remained largely unchanged, many of the smaller passages around the cave’s edges had shifted or disappeared.
For caves made of ice sitting on top of a volcano, they are “remarkably stable,” the authors wrote. “Well, stable-ish,” Florea says.
The caves on Rainier may be stable because the bowl-shaped crater protects the glacier from too much melt while the volcano provides just the right amount of heat, says Erin Pettit, a glaciologist at Oregon State University who has studied Rainier’s glaciers but was not a part of these expeditions. “It’s not surprising to me that these have been stable through time, as long as the volcano has been stable,” she says. “But it’s really neat to be able to see the longevity, knowing how sensitive glaciers can be.”
The map is all the more exciting because of the contributions of citizen scientists, says Jason Gulley, a University of South Florida geologist who was not involved in the work.
“The most incredible, most impactful part of all of this is that it’s exploration where a group of average citizens are out doing it,” Gulley says—although he points out there’s nothing “average” about the skills needed to work in some of the highest ice caves in the world.
The future of Rainier’s ice caves
The stability of the caves contrasts with two of Rainier’s neighbors: Mount Hood and Mount St. Helens. Sandy Glacier on Mt. Hood is in retreat, and its ice caves are shrinking along with it, while Mount St. Helens’ glacial caves are growing, as Stenner and Sobolewski reported in 2023.
It’s a tenuous balance, and with other glaciers around the world succumbing to climate change, scientists aren’t certain what will happen to the Rainier caves. “Each [volcanic glacier] will respond differently,” Stenner says. “We wouldn’t want these environments to disappear because they’re so unique.”
The new map will serve as a critical baseline for tracking changes, both from climate and volcanic activity. If the melt rate increases, it’s not just the caves that could collapse. The rock at the tops of volcanoes is a “crumbly mess” from the hot, acidic fluids circulating through them, and glacial ice is key for keeping the mountaintop intact, Pettit says. If the glacier goes, parts of the volcanic edifice could go, too, triggering deadly mudflows called lahars that could put thousands of people at risk.
Newly mapped, the caves will also be a valuable analog for other planets, Gulley says. “There’s a lot of work that astrobiologists can do inside these caves that has applications for understanding what life might be like on ice worlds like [Jupiter’s moon] Europa,” he says.
Climate change threatens to upset the balance of these ice caves, but for the time being, the frozen recesses of Mount Rainier don’t seem to be going anywhere.
“Everything is ephemeral, really,” Pettit says. “But it turns out, ice is kind of hard to melt.”
The National Geographic Society, committed to illuminating and protecting the wonder of our world, funded Explorer Christian Stenner’s work. Learn more about the Society’s support of Explorers at natgeo.com/impact.
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