The scientists prepped for their descent into the maw of the Greenland ice sheet by drilling deep into the ice. They created two intersecting holes into the bed of a now frozen-over ice river, running a rope through them in the shape of a V to anchor their lines. It would be more than strong enough to carry their weight, but they drilled a second anchor as well — just in case.

Then Matt Covington, a geologist and cave explorer who has spent more than a year of his life beneath the ground, was ready. He began to lower himself into the vertical cavern that, in the summer, fills with the chaos of a waterfall — a moulin. The sharp crampons on his boots gripped the ice. The fact that it was October now made the moulin a little safer, but Covington could still hear running water somewhere.

Below him, as he backed over the edge and looked down the shaft, he saw white ice, then bluer ice, then darkness. The hole, scientists believe, ultimately penetrates more than a half-kilometer into the ice, joining a network of channels extending all the way to the base of the ice sheet.

“When you’re dangling the wall of this deep shaft and you’re hearing ice breaking and falling, it brings your heart up into your throat,” remembered Covington, a professor of geology at the University of Arkansas.

Covington and his colleague Jason Gulley, an expert on ice caves at the University of South Florida, were motivated by a scientific question with enormous implications as the climate warms. Just how vast are these moulins, these ice caves found by the thousands across Greenland’s surface? How much are they undermining the integrity of the second-largest sheet of ice on the planet? And how much worse will it get as melting, and moulins, begin to extend farther and farther toward the airy center of Greenland, where the ice is well over a mile thick?

As Covington noted, he and Gulley operate from an old model of science — one where, as in the 18th and 19th centuries, exploration and research were one and the same. Darwin and other scientists merely had to travel around the world’s surface to make pivotal insights. But today, to find unexplored places, you have to get a little more extreme.

In the summer, Greenland’s moulins swallow up an ever-growing profusion of meltwater, which spills violently into them. Because the Greenland ice sheet extends high into the air, and has a complicated topography, the meltwater cannot simply run off its edge to the sea. Rather, it runs underneath the ice through moulins and furtively toward the ocean — where it spills out and raises the sea level.

“These are big giant scary holes in the ice,” Gulley said. “Places that historically people have avoided. During the summer you have entire lakes that disappear down these things, sometimes overnight. Almost every river on the Greenland ice sheet disappears into these holes.”

As the climate warms — with the Arctic warming fastest of all — more and more of Greenland’s surface is melting in the summer. More lakes are forming and at higher, colder elevations on the ice sheet. The ice sheet has lost some 4 trillion tons of mass just since 1992, and scientists estimate that surface melting makes up about half of those losses, with the rest being driven by huge icebergs calving into the sea.

Scientists have long known that Greenland’s meltwater uses thousands of moulins to escape, and inferred that these holes must be the surface entrance points to an enormous sub-ice network of drainage channels. And they may be destabilizing Greenland in the process.

“If you keep the crevasse full of water, water is more dense, it is able to exert a force and it basically rips the crack open all the way to the base of the ice sheet,” Gulley explained. “The water is acting like a wedge, that you smack with a hammer, to split firewood. So once you establish that hydraulic connection, the water has access to the base of the ice sheet and it starts squirting out everywhere.”

There has been a fear that as more and more water pumps into this network, the ice sheet itself may move faster toward the sea. That’s because the water pressure pushes outward on the ice, even as water at the base of the ice sheet lifts it up and helps it slide forward. It is already clear that, each summer melt season, Greenland’s ice speeds up in its movement toward the sea.

Countering that fear, though, has been the idea that as more water spills into these ice caves, deeper channels in the ice will open, releasing some of the hydraulic pressure and lessening the sliding. As a result, scientists are debating just how much moulins, and their accompanying water channels, will worsen sea level rise.

That’s where the research by Covington and Gulley comes in.

In 2018 and 2019, the scientists, first supported by the sports drink manufacturer Red Bull and later the National Science Foundation, traveled to a western sector of the Greenland ice sheet and rappelled into these scary features in the ice. While moulins are found in many glaciers in the world, Greenland hosts the biggest known examples.

“There are cracks in the walls, and one of the things with ice, it’s a pretty fragile substance,” Gulley said. “So if you have these big voids with lots of unsupported ice, it starts cracking … when we’re descending, we’re checking out the walls and making sure at no point we’re going to rig into something that could collapse.”

As the scientists dangled and took measurements using a device called a laser rangefinder, which reflects a laser beam to measure distance, they also had to contend with the elements.

“There was nowhere to really get off the rope,” Covington said. “For hours at a time, we were hanging on our harnesses in these ropes. It was cold and miserable for sure.”

The scientists were experienced in both exploring caves and also, caves in ice — and Will Gadd, a famed ice climber, was part of the first, Red Bull-sponsored trip. Still, Gulley admits that there was an element of danger involved that could not be fully controlled even by his considerable experience.

“There’s any number of things that could go wrong when you’re driving your car,” he said. “It’s the same thing when we’re exploring glacier caves, except that when we started doing this 20 years ago, nobody knew what was dangerous. Now, we know quite a bit about what’s dangerous. … So I feel like we’ve done a good job managing all the risks, but it’s never going to be as safe as sitting on your couch.”

In a few tragic cases, scientists working in polar extremes to study climate change have been killed in accidents. Most recently, this summer, renowned Swiss glaciologist Konrad Steffen died when he fell into a crevasse in Greenland.

Human exploration is necessary in the case of the moulins, Gulley said, because it is extremely difficult to navigate drones in such small and unpredictable spaces. And beneath the surface of the ice sheet you can’t use GPS to guide them. Plus, he noted, sending a drone just isn’t the same as being in a cave in person, seeing an interesting feature, and getting closer to try to study and understand it.

The first results of these moulin descents have been published in the journal Geophysical Research Letters. They combine observations of the breathtaking extent of the caves with scientific modeling of the water levels contained within them, and what that might mean.

The key finding: Moulins can be huge.

In particular, the Phobos moulin in western Greenland, which Gulley and Gadd explored in 2018, was not simply a narrow hole penetrating downward. Instead, it opened into a vast cavern that reached nearly 100 meters in depth before the water level began, and extended horizontally outward as well. The group calculated that, at the water’s surface, the spatial area of the cave was some 5,000 square feet, or the size of several houses next to one another.

This volume is much larger than previous models assumed. And it suggests that the moulin can store much more water than previously thought. This, in turn, might mean that the water in the moulins can exert more pressure on the surrounding ice and cause it to slide faster — which would be bad for sea level rise, and Greenland’s future.

“If the water levels aren’t fluctuating as much and they stay high, that means the ice sheet can continue moving,” Gulley said.

As climate change continues, more melting is expected atop Greenland, along with the formation of more moulins — including, possibly, even deeper ones. That’s because the melt will extend further inland toward the most elevated, and traditionally the coldest, parts of the ice sheet. Understanding how moulins work to bring water to the sea, and how they affect the speed of the ice’s movement, will be critical, scientists say.

“There is evidence of them forming farther and farther from the margins,” Covington said. “Not only is the melt moving inland but it seems that the processes that develop new moulins are marching inward.”

Granted, the researchers have only extensively explored two moulins so far, making it hard to generalize about thousands more that pockmark the Greenland ice sheet.

“Since this study is only able to examine two moulins in one region of the ice sheet, I think it’s too soon to say whether this finding might change our perspective on future variability of ice motion,” said Twila Moon, a Greenland expert at the National Snow and Ice Data Center in Colorado who was not involved in the research.

“The finding that larger-than-expected moulins are present in Greenland is significant, as these moulins provide a buffer between surface melting and the subglacial drainage system,” added Ádam Ignéczi, who studies moulins at Sheffield University in the United Kingdom and also was not involved in the latest research.

But Ignéczi added that scientists don’t know if “systematic differences” in the size of moulins can be found across the enormous ice sheet.

That may mean that scientists will have to study more moulins in coming years — carefully, of course.

“These are really beautiful places,” Gulley said. “As you drop in, you’re in this intensely blue chamber. And you can see all the layering of the ice. You’re basically looking at the inside of the Greenland ice sheet.”