Here’s the video:

The above footage debuted in a brief BBC report, which didn’t say much about what was actually happening in the video. So here’s a brief guide: What you’re looking at is warm ocean water working its way under Thwaites Glacier. The icy “ceiling” in the video is the bottom of the glacier’s ice shelf, the part of the glacier that floats in the water rather than sits on the seafloor. It’s more than 1,900 feet thick, and almost everything attached to the bottom of it was, only a few hours earlier, pressed into the Antarctic bedrock.

Read: A radical new scheme to prevent catastrophic sea-level rise

“The glacier is moving [several] meters a day, so that material we’re seeing at the grounding zone is brand new and is just exposed to the ocean,” Britney Schmidt, a glaciologist at the Georgia Institute of Technology, told me. She spoke by phone from McMurdo Station, the small American research base on the coast of Antarctica.

“We can definitely see it melting,” she said. “There are a few places where you can see streams of particles coming off the glaciers, textures and particles that tell us it’s melting pretty quickly and irregularly.” There’s also a moment, early in the video, that appears to show a barrier between the fresher water that’s coming off the ice shelf and the saltier water of the open ocean.

About 100 feet away from where the above video was shot, the glacier actually sits on the seafloor. The Icefin team hasn’t released video of that area yet, but they have released this still image:

Thwaites Glacier is a scientific twofer. It is important, first, because it is huge. It contains enough fresh water to raise global sea levels by more than a foot and a half, and it braces the entire West Antarctic Ice Sheet, which could raise sea levels by almost 10 feet if it pooled away. But Thwaites is also important because it is physically mysterious. In its enormous size and ominous future rest the answers to some of the biggest unresolved questions in climate science. As Andrea Dutton, a geologist at the University of Wisconsin who was not connected to the new research, told me by email: “Understanding the dynamics where the Thwaites is pinned to the sea floor is crucial to predicting the future of the world’s coastlines.”

One of the biggest mysteries is more than 40 years old. In 1978, the British glaciologist John Mercer noticed that West Antarctica, which looks like a solid plain of ice from space, does not sit on a stable foundation. If you took all the ice away, he said, West Antarctica would look more like an archipelago of little rocky islands, and its seafloor would slope down toward the South Pole. That may sound like a geographic curiosity, but in a world like ours, it tees up a potential calamity.