On the ice-covered edge of a remote West Antarctic bay, the continent’s most imperiled glaciers threaten to redraw Earth’s coastlines. Pine Island Glacier and its neighbor Thwaites Glacier are the gateway to a massive cache of frozen water, one that would raise global sea levels by four feet if it were all to spill into the sea. And that gateway is shattering before our eyes.

Over the weekend, the European Space Agency’s Sentinel satellites spotted a significant breakup, or calving event, underway on Pine Island Glacier’s floating ice shelf. A series of rifts that satellites have been monitoring since early 2019 grew rapidly last week. By Sunday, a 120 square-mile chunk of ice—a little under three