This is a story about a curious seal, a wayward robot, and a gigantic climate change disaster that may be waiting to happen.

Scientists tagged a southern elephant seal on the island of Kerguelen, an extraordinarily remote spot in the far southern Indian Ocean, in 2011. The seal was a male close to 11 feet long weighing nearly 1,800 pounds, and they fitted his head with an ocean sensor, a device that these massive seals barely notice but that have proved vital to scientific research.

Elephant seals like this one swim more than 1,500 miles south from Kerguelen to Antarctica, where they often forage on the sea floor, diving to depths that can exceed a mile below the surface. As summer in the Southern Hemisphere peaked, the seal made a standard Antarctic journey, but then went in an unusual direction.

In March 2011, he appeared just offshore from a vast oceanfront glacier called Denman, where elephant seals are not generally known to go. He dove into a deep trough in the ocean bed, roughly half a mile below the surface. And that is when something striking happened: He provided an early bit of evidence that Denman Glacier could be a major threat to global coastlines.

The seal swam through unusually warm water, just below the freezing point, but in the Antarctic, that is warm. Given its salt content and the extreme depths and pressures involved — in some regions Denman Glacier rests on a seafloor that is over a mile deep — such warm water can destroy large amounts of ice. And it certainly could have been doing so at Denman.

Yet scientists do not appear to have seen the significance of the seal data. Back then, Denman had not received much scientific attention. It did not help that the glacier is extraordinarily difficult to study directly. It lies between the two Antarctic research bases of Australia. The logistics are challenging for a voyage from either side, especially as the glacier is often locked in by extensive sea ice.

Researchers had already observed that the glacier was losing some of its mass, which is a worrying sign. They also knew something else: Denman serves as a potential doorway into a region of extremely deep and thick ice, even for Antarctica.

With Denman and several other neighboring glaciers in place, the doorway remains closed. Opening it would allow warmer ocean water to start eating away at this thick ice, leading to gradual melt and eventually, a massive influx of new water into the ocean. That would have the potential to unleash over 15 feet of sea level rise, remaking every coastline in the world. So the scientists flew a few planes over Denman and watched with their satellites. And they waited.

The robot

A striking discovery came in 2019. Using satellite data and other techniques, scientists published a new elevation map of all the crushed-down land beneath the Antarctic ice sheet. And it showed that beneath Denman lay the deepest point of them all, about the depth of two Grand Canyons, or two miles below sea level. If water, rather than ice, were to someday fill this valley, Denman Glacier could raise global sea levels by nearly 5 feet.

Almost simultaneously, scientists reported something else: Denman was reeling. The region of its “grounding line,” where the glacier touches both the seafloor and the ocean, had retreated backward more than three miles toward the center of Antarctica since 1996, bringing the sea to the edge of the newly discovered canyon.

It was in this context that researchers now unearthed the nine-year-old measurements from the seal. “We dug out these data because we wanted to find out if warm water can indeed reach this glacier,” said Eric Rignot, an Antarctic expert at the University of California at Irvine and one of the authors of the paper. “The answer seemed to be yes.” But while the seal sensor proved the presence of warm water, it did not reveal how much of it might be hitting the glacier.

Enter the robot.

A group of scientists based in Australia, led by Esmee Van Wijk of the Commonwealth Scientific and Industrial Research Organisation, were trying in 2020 to study Totten, a gigantic glacier hundreds of miles from Denman.

Denman and Totten are two main gateways into one of the largest lodes of thick ice in Antarctica. They both sit atop deep channels leading into the Aurora Subglacial Basin, an enormous inland region of East Antarctica where the ice largely rests below sea level, sometimes more than a mile deep. If the ocean reached here it would be catastrophic, making the sea level rise from a full melt of Denman look small.

Van Wijk and her fellow scientist and partner Stephen Rintoul weren’t actually present at Totten — they were at home in Tasmania. But a scientific vessel in the region deployed their research tools called Argo floats. These clever robots dive to great depths, taking temperature and other readings periodically, and then surface again as long as there is not any ice above them. Then they radio the data back to the humans who are anxiously awaiting it.

But not unlike that seal a decade earlier, one of the floats wound up in an unexpected place. It was carried off course by currents but, eight months later, fortuitously surfaced in front of Denman Glacier. It appeared “in a region we really wanted to sample, but is very difficult to sample with ships, it is often covered with quite heavy sea ice,” said Van Wijk. “So for us, it is a case of getting very lucky.”

The robot was a little more thorough in its explorations than the seal. It also measured water that was even warmer at very, very close to zero degrees Celsius. Thanks to these measurements, the scientists were able to determine just how much of this warm water is flowing toward Denman Glacier. It was massive.

There are “about eight Mississippi Rivers going into the cavity,” according to Rintoul. The scientists calculated, in a paper recently published in Geophysical Research Letters, that with this volume of water and its temperature, there is the potential to melt 71 billion tons of ice from the underside of Denman Glacier, where its “ice tongue” floats out over the ocean, every year.

The warm water in question is technically called “circumpolar deep water.” It encircles the Antarctic within the middle depths of the ocean, but recently, for reasons that may have to do with changing patterns of winds, a likely result of climate change, the circle around the continent appears to have grown tighter.

As a result, the warm water has been increasingly climbing onto continental shelves and assailing glaciers at their weak points: their bases where they rest on the bedrock. “You can think of it like a blanket that is draped across the seafloor,” said Van Wijk of the water layer.

The increased interest in Denman is significant because scientists have long been focused on West Antarctica. It is well-known that this warm water has been melting the Thwaites and Pine Island glaciers, causing them to spill billions of tons of ice into the ocean every year.

While East Antarctica has so far not been losing much, if any, ice, it has much more to potentially give than any other Antarctic region. Like these West Antarctic glaciers, Denman also has a dangerous configuration. It has a “retrograde” slope, which means that the glacier gets thicker, and the seafloor slopes downward, as you move inland from where the ice sits.

Glaciers in this position are prone to rapid retreat called “marine ice sheet instability,” and it makes a kind of intuitive sense. As a glacier perched on such a slope begins to melt at its base and move backward downhill, more of its surface area is exposed to the ocean. That increases both the ability of the ice to flow outward and the extent to which the ice can melt.

The humans

Based on measurements from the robot, Van Wijk and her colleagues can confirm that a lot of warm water is heading toward Denman. But they do not know what happens after that. There are complicated seafloor contours, including several shallower ridges, that the water must traverse before reaching the ice.

Considering that Denman has been retreating, scientists are operating from the working assumption that some of the warm water is getting all the way to its base. Several experts said the new research is an important step forward, but there is much further to go.

While satellite images of massive loss of the West Antarctic glaciers have taken up much of the scientific community’s attention, East Antarctic glaciers are vulnerable to the same mass loss, said Helen Amanda Fricker, an Antarctic expert at the Scripps Institution of Oceanography who was not involved in the study. The East Antarctic basins are “starting to show signs of change,” she said, and the new paper shows there is “enough heat flux in the ocean” to drive melting of some of the very large glaciers in the region.

There is still a great deal of uncertainty about what Denman will do next, and scientists — hobbled by how little they know about this region — cannot predict it at this point, said Don Blankenship, an Antarctic expert at the University of Texas at Austin. “The ocean is delivering its heat, and now we need to ask the question, what is that heat going to do in the interior?” he said.

The precise details of what kind of bedrock the ice sits upon, and the exact contours of the ocean floor and the rock walls surrounding the wide glacier, will matter. These will determine how rapidly Denman, which is 10 miles wide, can slip out of place and start retreating further backward into the canyon, and whether it will get stuck anywhere along the way. And scientists just do not know many of those details.

“Denman is on the want-to-do list for everybody,” Blankenship said. Australian researchers are planning a Denman expedition aboard their country’s new icebreaker, but that will not happen until the Antarctic summer in 2024. The German research vessel Polarstern is also scheduled to reach the glacier next year.

One of the most disconcerting things about climate change is that what we do not know may hurt us the most. When it comes to Denman, said Van Wijk, “we probably know more about parts of the moon.” It is thanks in part to good fortune that we know as much as we do. We have had news from a seal and a robot, but it looks like it is time to send in the humans.