By Chris Mooney and 

Simon Ducroquet

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.

A southern elephant seal takes a peek as it swims in the icy waters of Antarctica. (Sam Eves/Shutterstock) 
A satellite image of the Denman region in Antarctica. (Landsat Image Mosaic of Antarctica Project) 

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 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.


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