In 1945, a year after the Allies stormed the beaches of Normandy, the ocean was busy establishing a beachhead of its own, burrowing beneath a fortified section of an important Antarctic glacier. Pine Island Glacier, a Texas-sized, 2-kilometer-thick ice sheet (pictured), is the linchpin of the rapidly disappearing West Antarctic Ice Sheet—one of the largest drivers of uncertainty for sea level rise this century. No glacier has lost more water to the ocean in recent years: It is thinning by more than a meter each year as warm ocean water creeps in and melts it from underneath, hollowing out a vast ice shelf. Now, scientists can trace the beginning of this accelerated melting to a surge of warming in the Pacific Ocean more than 70 years ago. Researchers already knew that in the 1970s the glacier lost contact with an undersea ridge that had held ocean water at bay. But how long did it take for the ocean to worm its way through? Working in remote conditions, researchers in the winter of 2012 ran a drill through 450 meters of ice and 500 meters of ocean to collect seafloor sediments on either side of this lost bulwark. Analyzing and dating these rocks, they found that ocean water began to appear on the ridge's land-facing side in 1945, even as the ice sheet remained grounded on the ridge’s summit, scientists report online today in Nature. Furthermore, they found that the incursion of ocean water followed a notably warm El Niño in the Pacific Ocean between 1939 and 1942. It would be nearly a half-century before the oceans around Antarctica saw such warmth again. Yet the water in the cavity never refroze, suggesting that the melting of some ice sheets will be difficult to reverse, even if human-driven warming is curbed.