Temperatures on the West Antarctic Ice Sheet can plummet below –50 degrees Celsius in winter. But under the ice scientists have found intense geothermal heat seeping up from Earth’s interior. The heat production that they measured is nearly four times the global average—“higher than 99 percent of all the measurements made on continents around the world,” says Andrew Fisher, a hydrogeologist at the University of California, Santa Cruz, who worked on the project. This excessive heat could melt up to 35 cubic kilometers of water off the bottom of the West Antarctic Ice sheet each year, according to results reported July 10 in Science Advances.

This meltwater could help create a vast, hidden habitat for aquatic life under the ice—a region that some scientists call the largest swamp on Earth. It could also influence the mechanics of the ice sheet by creating lubricated areas, which guide the flow paths and speeds of major glaciers that carry ice to the ocean. “We think that water is the knob that controls whether ice moves fast or slow,” says Slawek Tulaczyk, a glaciologist at U.C. Santa Cruz. Scientists like him need to understand that process if they are to predict just how much ice Antarctica will spill into the ocean as temperatures rise.

Researchers had already measured geothermal heat production at more than 34,000 sites around the world. But for decades, they could only make educated guesses about how much heat was seeping up under Antarctica’s ice—an area almost twice the size of Australia that had never been directly explored. That changed in January 2013 when a team co-led by Tulaczyk ventured deep into Antarctica and bored a hole through 800 meters of ice.

Tulaczyk’s team was drilling into a body of water called Subglacial Lake Whillans, which is sealed under the ice in West Antarctica—the part of the continent that sits directly south of the Pacific Ocean, between the lowermost tips of New Zealand and South America. The team hoped to see what kind of life might inhabit the lake. Their experiment also created the perfect opportunity for jabbing a giant thermometer into its bed—a metal spear, three meters long, accurate to one one-thousandth of a degree C. Fisher had spent two years building and testing the device. The critical moment came on January 31, 2013. The entire endeavor hung from a thread—or rather, a hastily knotted rope.

Geothermal night-light
I accompanied Tulaczyk and a dozen other researchers to the remote drill site that year. It sat on a monotonous plain of snow and ice 600 kilometers from the South Pole. Tulaczyk blinked in the brilliant sunlight as he crawled out of his tent around 1:00 A.M. that morning. He quickly received bad news: The massive winch that he needed to lower the probe half a mile into the lake had broken down. The probe weighed 550 kilograms, more than a full-grown horse—a hefty mass that would help drive it into the subglacial mud. But now Tulaczyk and his PhD student, Kenneth Mankoff, spent eight frantic hours disassembling and redesigning it to cut its weight in half so that a smaller winch could handle it.

This slimmed-down version of the thermometer had one major drawback: It now had no ring of any sort that would allow them to clamp it onto the end of the winch’s cable. In fact, it consisted of nothing more than a slender metal pole. They improvised by simply knotting a rope around its smooth shaft—a workaround that seemed destined to fail. “It either works or it doesn’t—it’s a one-time thing,” Tulaczyk told me during a brief break that morning. He worried that the rope would pull off the metal probe when they tried to lift it back up—leaving it jammed for eternity in the viscous mud almost a kilometer below.

And so people were understandably relieved when it was hoisted from the borehole several hours later. The chocolatey mud coating its lower half revealed that it had stabbed more than a meter into the lake bed—just enough to measure the heat flow.

The results published on July 10 show that heat energy is seeping out of the planet at a rate of 285 milliwatts per square meter. That’s a tiny amount of energy—equal to the heat dissipated by a small LED night light. But it’s two or three times the amount of geothermal heat that scientists had previously estimated for West Antarctica. In fact, it’s similar to measurements taken in volcanically active areas, such as Yellowstone in Wyoming and Mount Lassen in California. This higher heat measurement, if multiplied across all of West Antarctica, could liberate an extra 10 to 20 cubic kilometers of meltwater under the ice sheet each year—effectively doubling the amount produced.

Slick and slide
“This is one measurement,” Fisher cautions—the heat flux probably varies from place to place. But that raises some fascinating possibilities: The area where they measured geothermal heat contains half a dozen subglacial lakes, with water flowing from one to another through shifting, braided subglacial rivers. Those lakes might owe their existence to a local geothermal hotspot, which supplies them with water, Fisher says. Hotspots might also account for many of the other 60-plus lakes thought to reside under the ice in West Antarctica.

Those lakes are of great interest due to the aquatic life that they might harbor. Water taken from Lake Whillans (where Tulaczyk, Mankoff and Fisher measured heat flow in 2013) was found to hold 130,000 living cells per milliliter (just over half a million per teaspoon)—a surprising amount, similar to some parts of the open ocean.

West Antarctica probably exudes more heat than the higher-elevation eastern part of the continent, Tulaczyk says. Unlike its eastern counterpart, West Antarctica forms a broad, low saddle; its subglacial bed slopes hundreds, even thousands of meters below sea level. This topography was formed by a broad tectonic rift, he adds, “similar in many ways to the Basin and Range [Province] in Nevada and eastern California.” Gradual stretching has thinned Earth’s crust in this region, allowing hot rocks and magma to bulge up from below.

Much of the ice in Antarctica’s interior creeps only a few meters per year—but a handful of major ice streams, up to 100 kilometers across, flow hundreds or even thousands of meters per year—massive conveyor belts that carry hundreds of cubic kilometers of ice out of the interior yearly and dump it into the ocean. Tulaczyk and others believe that local hotspots influence the flow paths and speeds of these ice streams.

In 2014 scientists reported that one major West Antarctic ice stream, called Thwaites Glacier, sits atop several local hotspots (inferred using ice-penetrating radar and computer modeling). These could melt water and lubricate the glacier, says Duncan Young, a glaciologist with the University of Texas at Austin who was part of that study. The hotspots sit beneath several critical spots in the glacier’s inland tributaries, potentially increasing the supply of ice that is poured into the main trunk of the glacier—and eventually, the ocean, where it contributes to sea level rise. Thwaites Glacier is of particular interest because it is already accelerating and thinning in response to rising temperatures.

Much remains to be learned about the vast landscape hidden beneath the West Antarctic Ice Sheet but one possibility is becoming increasingly clear. Aerial surveys using ice-penetrating radar show numerous isolated high spots in the subglacial topography. These often correspond with strong magnetic anomalies—a marker of iron-rich lava rocks. “There have been at least three subglacial volcanoes identified under the ice sheet now,” Young says—“and we have suspicions of a bunch more”—perhaps hundreds. Dozens of these suspected volcanoes possess unusually squat profiles, suggesting that they actually erupted and grew while buried under the crushing weight of the ice sheet. At least one subglacial volcano is thought to be active right now—a submerged peak named Mount Casertz.

The upper surface of the ice sheet dips 50 meters as it flows over the buried crest of this volcano. Maintaining that low spot year after year is no small thing, because the crushing mass of the surrounding ice sheet should ooze inward and fill even a shallow depression. Calculations suggest that Mount Casertz exudes 700 million watts of geothermal heat—roughly equal to the energy produced by a medium-size nuclear power plant. It maintains the topographic depression above it by melting 70 million tons of water off the bottom of the ice sheet each year.

It’s entirely possible that Casertz or another of these hidden volcanoes could erupt in the future. No one believes that even a catastrophic eruption would rip apart the ice sheet—it’s simply too massive. But the meltwater that it produces could still cause a large glacier like Thwaites to speed up in a way that’s never been seen before.

Young and his colleagues in Texas continue to analyze radar and magnetic data, hoping to assemble a clearer picture of volcanoes under the West Antarctic Ice Sheet. “We haven’t looked everywhere,” Young says. “Our resolution of the topography [under the ice] is basically early 20th century, maybe 19th century, of what we had of North America.”