Deploying an underwater robot beneath a rapidly melting ice shelf in Antarctica, scientists have uncovered new clues about how it is melting. The findings will help assess the threat it and other ice shelves pose for long-term sea-level rise.

The researchers said that overall melting of the underside of part of the Thwaites shelf in West Antarctica was less than expected from estimates derived from computer models. But they also discovered that rapid melting was occurring in unexpected places: a series of terraces and crevasses that extended up into the ice.

The findings do not alter the fact that the Thwaites is among the fastest receding and least stable ice shelves in Antarctica, and of the most concern when it comes to sea level rise. It also does not change forecasts that the collapse of the shelf and the glacier it is part of would lead to about two feet of rise over several centuries.

The research “is telling us a lot more about the processes that drive retreat on Thwaites,” said one of the scientists, Peter E.D. Davis, an oceanographer with the British Antarctic Survey. The findings, published Wednesday in the journal Nature, will be used to refine models that forecast Thwaites’s long-term future.

The research is part of a large effort, the International Thwaites Glacier Collaboration, sponsored by the United States and Britain, to better understand what is happening at the Thwaites.

The ice shelf is the floating tongue of the Thwaites glacier, a Florida-sized river of ice that helps to hold one of Antarctica’s two massive ice sheets in check. The waters surrounding Antarctica are warming as a result of climate change, and as this warm water flows under the shelf, the ice melts from underneath and the shelf becomes thinner. The so-called grounding line, the area where the floating ice meets bedrock, has been retreating as the shelf loses ice, moving about 8 miles inland over the past two decades.

The Thwaites already contributes about 4 percent to the current overall rate of global sea level rise of about 1.5 inches per decade. Its retreat has accelerated in recent decades, but whether it is at or near the point where its collapse is inevitable is a subject of debate among scientists. If all the main glaciers in West Antarctica were to collapse, they would add 10 feet to sea-level rise over thousands of years.

Ted Scambos, a senior researcher at the University of Colorado Boulder, said the new findings, and other recent work on the Thwaites, suggest that although many uncertainties remain, the worst-case scenario for the ice shelf, at least this century, “is a little less worse than it used to be.”

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“We’ve kind of shrunk the monster a little bit,” said Dr. Scambos, who is part of the Thwaites effort but was not directly involved in this research.

The new findings were in two papers in Nature: Dr. Davis was the lead author of one, and Britney E. Schmidt, a geophysicist at Cornell University, was the lead author of the other.

The researchers camped on the ice during the Antarctic summer of 2019-20, often in extreme cold and windy conditions, and used hot water to bore several holes through 2,000 feet of ice to the ocean below not far from the grounding line.

Dr. Davis and his team lowered instruments into the water to measure its temperature, salinity and other characteristics. While they found that the water was substantially above the freezing point, the slow current and the layering of water of different salinity levels prevented mixing that would have brought more heat upward and melted more ice.

Alastair Graham, an oceanographer at the University of South Florida who has studied the historical retreat of the Thwaites ice but was not involved in these two studies, said that the work by Dr. Davis’s team showed that “there is plenty of heat making its way all the way up to Thwaites grounding zone.

“However, not all of that ocean warmth is turned into melting,” he said.

The star of the show was the underwater robot, called Icefin, which was designed, built and operated by Dr. Schmidt and her team. A cylinder 9 inches in diameter and about a dozen feet long, it carries cameras, sonar and other instruments, as well as thrusters for propulsion. Dr. Schmidt slowly “drove” the device via a long tether that carried signals from the surface.

“Getting to see the ice for the first time was really powerful,” Dr. Schmidt said. “There were some really intense experiences.”

Among them was driving the robot toward the grounding line, where the water column between the ice and bedrock narrowed to barely more than the diameter of Icefin itself. Squeezing into that space “was pretty remarkable and very exciting,” she said. “And it was also terrifying.”

Icefin explored crevasses and steep-sided terraces on the underside of the ice, and found rapid melting there, as the near-vertical orientation of the sidewalls allowed mixing and brought more heat to bear on the ice.

At times, Icefin allowed the researchers to measure what was occurring within just a few inches of the ice. Seeing those ice faces and their orientation up close was perplexing, she said, “and trying to figure that out has been a big part of the story.”

Like Dr. Davis, Dr. Schmidt said that the findings provided important context for what is happening at the Thwaites glacier. “It’s not ‘warm water equals X amount of melting,’” she said. “It’s ‘warm water plus process X means melting.’”

Because overall there is less melting on the underside but the Thwaites is still unstable, she said, “it means it actually takes a lot less than we thought to push these things out of balance.”

“It doesn’t mean things are better,” Dr. Schmidt added. “It means that things are different.”

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