All scientist Erin Pettit could see when she looked at the satellite photos of the ice shelf in front of the Thwaites Glacier in West Antarctica was the giant crack that stretched across most of the image.

Two years before, when she and her colleagues were deciding where to put their research camp, the entire floating ice shelf—a tongue of ice poking out from the enormous glacier behind it—was solid. It was plenty safe to plan a camp there, they thought.

But last December, when they were preparing to go to the camp, the images revealed enormous cracks in the ice pointing straight at it.

It was unlikely the cracks would grow fast enough to endanger them. But to Pettit, it signified something