The Airbus helicopter bucked wildly in the frigid air hundreds of meters above the Chilean Andes. Every time it heeled over to turn, the helicopter dropped 20 meters, and the jagged peaks below seemed to rush up at Brad Singer through a gaping opening in the copter’s side—the door had been removed to make photography easier. The jolts and the clear path to the ground did not make Singer, a geologist usually based on the flat land around the University of Wisconsin–Madison, feel very secure even though he was strapped into a seat. “It was like being in a big, loud open roller coaster,” he recalls.

But the awe-inspiring scene far below, with its landmarks of a vast volcanic history, distracted him from his worries about falling. At one point, Singer and his colleagues flew over a pale pink lava flow that stretched across the stark landscape for 20 square kilometers—a footprint that could easily hold 25 Mount Kilauea summit craters. And that was only part of what the geologist had come to see. This volcanic region, known as Laguna del Maule, hosts about 50 lava flows and 70 ash deposits, ringed around a 54-square-kilometer icy blue lake. Only from above, says Singer, who has been exploring this area for the past 20 years, can you truly grasp the region’s gigantic scale. Only then can you take to heart that Laguna del Maule hosts the world’s largest collection of recently erupted rhyolite, a rock that, in its liquid magma form, can be particularly explosive and dangerous.

Geologists suspect that the flows and deposits are the handiwork of 25 to 30 different volcanoes over the past 20,000 years. This record is eerily like a place called Long Valley in California, where, in addition to a spate of what Singer calls “business as usual” explosions, a single enormous volcano blew a 500-square-kilometer pit in the land roughly 765,000 years ago. That geologic similarity, plus an ominous swelling of the ground at Laguna del Maule that began a decade or so ago, suggests the region might be capable of becoming the world’s next supervolcano.

Supervolcanoes hold some of the most destructive forces on the planet. These eruptions send at least 1,000 cubic kilometers of rock and ash hurtling out of the earth at once—2,500 times more material than the devastating Mount St. Helens explosion in 1980. Underground maps of Laguna del Maule’s magma chamber have recently indicated it has grown large enough to put that amount of material in the air, should it all blow at once. Even if Laguna del Maule lets off some of its steam in a series of smaller eruptions, the system looks like a good model for the ancient giants.

Credit: Mapping Specialists

It also has one key feature all other known supervolcanoes lack: it is forming in front of our eyes. Until today, everything that geologists know about these behemoths has been gleaned from old rocks and ash deposits and other aftereffects—forensic work that allows them to reconstruct a few aspects of the explosions. But Laguna del Maule is rousing right now, offering the first live look at the way such catastrophes develop. “We have no human experience with these kinds of large supervolcanic eruptions,” Singer says. (The prospect of human experience is a major concern for the several hundred thousand people who live in nearby cities and towns. While a supereruption does not seem likely any time soon, government agencies in Argentina and Chile are keeping a close eye on the region.)

That up-close peek at Laguna del Maule, along with findings from older supervolcanoes, has already pushed scientists toward a surprising conclusion: the giant underground chambers of magma that fuel these monsters are not the hot vats of molten lava that researchers once imagined. Instead the mass is cold enough that it is often solid. The realization has presented volcanologists with a puzzle: for an eruption to occur, solid magma has to melt and rise rapidly, within a matter of decades, so geologists have been trying to explain how this sudden change from cold to superhot happens.

Recently Singer and some of his colleagues have seen signs that the cool magma reservoir may be hit by a sudden jolt of hot bubbles of water from below; in other volcanoes, scientists have found evidence that magma from a lower, hotter chamber bursts into the higher, chillier one. As researchers try to piece together these clues, the real-time observations at Laguna del Maule might help them explain our planet’s biggest bangs.

A Rising Power

The forces unleashed by historic supervolcanoes were staggering. Roughly 631,000 years ago, for example, one erupted in what is now Yellowstone National Park in the U.S., sending lethal waves of hot gas, volcanic rock, ash and toxic fog racing across the landscape. Those eruptions filled entire valleys with material so hot and heavy that it welded into cliffs that are as thick as 200 meters today. Afterward the sky was dark with ash that went on to fall across an enormous swath of North America, leaving a layer of debris that spanned a triangle stretching from today’s Canadian border down to California and over to the Gulf of Mexico. At times the ramifications of a supervolcano can be felt across the globe. A few have created layers of ash so thick that scientists believe they blocked the sun’s light and plunged the earth into a volcanic winter.

No supervolcano has erupted in modern history. But in 2008 Matthew Pritchard, a geophysicist at Cornell University, was scouring data from a satellite when he noticed a surreal signal emanating from the Chilean Andes. The data on his computer screen revealed a psychedelic tie-dyed bull’s-eye. The ring pattern was typical for any changes in ground elevation. The odd thing was that rings such as these were usually located at the top of a single known volcano, but here the pattern was centered on a large region with hills and plateaus stretching about 400 square kilometers. Something unusual was happening. When Pritchard looked through earlier images, he found that the uplift, which must have begun between 2004 and 2007, was causing the ground to shoot upward by 20 centimeters a year. That is the largest rate of ground-level change anywhere in the world, 10 times larger than the “enhanced” uplift at Yellowstone that has occurred (and then stopped) a few times during the past several years.

Credit: Emily Cooper

That movement at Laguna del Maule, combined with the area’s explosive record, has drawn a number of expeditions in recent years to determine if the area is indeed getting close to an eruption and if so how large it might be. In 2013 Singer began a five-year project to study the system’s past and current state. Working with Chile’s National Geology and Mining Service, he and his colleagues deployed about 50 sensors on the ground. They flew helicopters above the lake that carried instruments to scan the surface below. And they pecked away at ancient rocks with a shovel and a hammer, removing samples that they could further analyze in the laboratory.

All the data indicated that something very big was brewing under the surface. “I don’t want to be an alarmist or anything, but it does suggest that there’s a developing larger collection of magma underneath Laguna del Maule,” says Judith Fierstein, a geologist at the U.S. Geological Survey’s California Volcano Observatory in Menlo Park, Calif. Clues about underground structures gleaned from the way vibrations travel through them, as well as changes in gravity and electrical conductivity produced by different rock types, all suggest the region sits on top of a reservoir holding 450 cubic kilometers of explosive rhyolitic magma. If it all erupts, that amount of magma can transform into the 1,000 cubic kilometers of ash, rock and lava needed to meet the minimum supervolcano definition, Singer says.

The magma does not all have to blow at once to be dangerous. Only 10 percent of that volume would produce an explosion about twice the size of Krakatoa, which, though not a supervolcano, killed 36,000 people in Indonesia when it blew in 1883.

Stone-Cold Killer

While some scientists measured Laguna del Maule’s size, others were more concerned with its temperature. In the classic view, the magma stored below active volcanoes is a seething liquid mass that then rises toward the top of the crust like a glob within a lava lamp. But in 2014 Adam Kent of Oregon State University and Kari Cooper of the University of California, Davis, made a startling discovery that prompted many in the research community to consider that some volcanoes really might be cold-natured.

VOLCANIC FEATURES around Laguna del Maule, seen in a satellite view, show vegetation (red) and snow-covered peaks (white). Credit: NASA, GSFC, METI, ERSDAC, JAROS and U.S. and Japan Aster Science Team

Kent and Cooper examined tiny crystals embedded within the volcanic rocks previously released from Oregon’s volcanic Mount Hood. Before those crystals erupted, they grew in the magma chamber below, accreting layer after layer that—much like a set of tree rings—recorded the evolving chemistry of the magma, including changes in composition, pressure and temperature. The crystals from Mount Hood rocks, for example, indicated that its magma spent as much as 99 percent of its time at temperatures too cool to erupt. It was not really a liquid but a kind of mush: a network of crystals with some fluid between them. “It’s kind of like peanut butter, you know, when you pull peanut butter out of the fridge and you try to spread it—it’s too solid,” Kent says.

That finding made scientists wonder whether the same story might hold true for other volcanoes. The answer appears to be yes. In 2017 Cooper and her colleagues performed the same analysis on crystals that had erupted from New Zealand’s Taupo Volcanic Zone—a region that runs down the center of New Zealand’s North Island and has seen several supereruptions. The scientists learned these crystals had also spent most of their lifetime nestled within cool, solid magma. And in late 2017 Singer and his colleagues analyzed the volcanic leftovers from the Long Valley supereruption in California and saw a similar record. Yellowstone’s magma chamber also holds a cooler crystalline mush.

Geologists place sensors near the lake to detect rising ground. Credit: Brad S. Singer University of Wisconsin–Madison

Although Kent and Cooper have found hints that magma might reside at slightly hotter temperatures at a few old sites in North and South America, it is becoming very clear that in general supervolcanoes tend toward the cold. And the behemoths stay cold until the moment before they erupt. That means that hot magma is ephemeral, only coming into existence immediately before eruption. Cooper’s work at Taupo suggests that these systems liquefy only 40 years prior to eruption. At Long Valley, the liquefaction happens in decades to a few hundred years. At Yellowstone, it also takes mere decades. Cooper says these timescales could run as short as a few years because scientists tend to publish conservative estimates.

What about Laguna del Maule? There, too, the massive magma body appears to be a crystalline mush. Singer and his colleagues argue that 95 percent of the system is composed of crystals, whereas a mere 5 percent is liquid melt. The mush is also a relatively chilly 800 degrees Celsius, more or less; lava that cascaded down the slopes of Hawaii’s Kilauea this summer, in contrast, was about 1,200 degrees C.* The discoveries reinforce the notion that these cold-hearted beasts wake up in an instant. When Nathan Andersen, a geologist at the University of Oregon, analyzed the small crystals embedded within the past lava flows at Laguna del Maule, he saw that they resided in magma for only 10 to 100 years.

Awakening A Giant

But that picture presents a conundrum: How do these frozen systems melt and mobilize so rapidly? At Yellowstone, the last great supereruption occurred when liquid magma rose up through the earth and hit an upper, solidified reservoir, contends Christy Till, a geologist at Arizona State University. Her model is a broad outline—there were no scientists around with sensors at the time—but it appears from volcanic leftovers that the rising liquid mixed with the more solid material, melting crystals. This heated the entire chamber, and the added liquid volume built pressure until the liquid exploded onto the surface.

What is happening in Chile, however, may be very different. “There isn’t a one-size-fits-all model for these volcanoes,” Till says. Yellowstone sits atop a hot plume, for instance; Laguna del Maule and Long Valley do not. At those spots, one plate of planetary crust is diving underneath another, melting rock into magma as it goes. That creates a different type of eruption trigger, Andersen says, one that heats only part of the volcano reservoir and involves an explosion of bubbles.

When Andersen analyzed crystals from Laguna del Maule’s past eruptions, he found that basaltic magma originating deep in the planet did not completely infiltrate the volcanic system. Such magma stalled at the base of Laguna del Maule’s upper rhyolite-rich reservoir. The two types of rock never mixed. The basalt cooled down when it stalled, releasing heat into the material above it. It also gave off volatile water vapor in the form of bubbles. As the heat and the bubbles started to rise, they melted some of the surrounding crystals, creating buoyant, bubble-filled plumes that percolated upward to the top of the reservoir. At that point, they exerted so much pressure on the crust above that they burst out.

This idea worries Singer. If volatile bubbles are currently rising below Laguna del Maule, there is no way for them to escape. There are no hydrothermal features, such as Old Faithful geysers, fumaroles, hot springs or vent sites, near the mountain lake. “In my mind, that makes Laguna del Maule potentially more dangerous than other kinds of systems,” Singer says. If some gases were able to leak to the surface, the growth of the reservoir might slow. “But if you’ve got this reservoir that’s trapped down there and incubating down there, this is probably the kind of condition that can allow a system to grow very large, like Yellowstone did before its last big eruption,” Singer says. “That would be a game changer. That’s the kind of eruption that humans haven’t seen.”

Thus, although the reservoir below Laguna del Maule is barely at supervolcanic size, it could grow even bigger over the coming centuries. At the moment, Singer’s fears are somewhat speculative. The eruption mechanisms are, too. It might be, for example, that rising, bubble-rich melt cannot create enough pressure by itself to generate a giant explosion and requires the help of a local earthquake. Singer’s team is currently trying to identify signals associated with such additional hidden triggers.

Even if Laguna del Maule were to release a series of smaller eruptions—and the record of ash and lava on the surface indicates that it has done that in the past—the effects could still be felt across South America. Ash would halt air traffic in the area and potentially devastate agricultural production for years within Argentina (where it will likely land because of westerly wind patterns).* Additionally, ash flows might breach a dam at the nearby Maule River, thus causing catastrophic flooding that could hit Talca, a city at the bottom of the valley that hosts more than 200,000 residents.

Singer’s team has removed its instruments from the field, but scientists from Chile and Argentina have picked up the baton, placing new monitors that help to watch over the lake. At the moment, the region continues to steadily balloon toward the heavens—movement that appears relatively harmless. But should the lake show further signs of unrest, researchers just might be able to accurately predict the volcanic field’s next eruption—now that they better understand the spark that can ignite a cold beast.

*Editor's Note (1/31/19): These two sentences from the print article were edited after posting to clarify the former and correct a directional error in the latter.