Detailed seafloor mapping of submerged glacial landforms finds that Antarctic ice sheets in the past retreated far faster than the most rapid pace of retreat observed today, exceeding even the most extreme modern rates by at least an order of magnitude, according to a new study. Using an autonomous underwater vehicle (AUV) to carefully measure the delicate sets of ridges left behind in seafloor sediments by retreating ice following the Last Glacial Maximum (LGM) roughly 14,000 years ago, the authors estimate deglaciation at a rate of more than 10 kilometers per year. The findings provide a clear indication of how quickly massive ice sheets can disappear into the ocean - events that, if repeated, would have significant implications for modern sea-level rise. Antarctica's ice shelves - the waterborne fringes of tidewater glaciers - form the boundaries between the Southern Ocean and the Antarctic Ice Sheet. At the grounding-line, the place where the ice sheet no longer rests directly on the sea floor and begins to float, warm air and ocean water conspire to melt ice from above and below. As a result, these regions are prone to rapid retreat. How the fastest rates of modern ice loss stack up against the maximum possible magnitude of retreat, however, is unknown. On the Larsen continental shelf off the east coast of the Antarctic Peninsula, Julian Dowdeswell and colleagues investigated a series of ridges in the soft seafloor sediment, which they interpret as artifacts left behind during the tidal rise and fall of the ice-shelf at the grounding line. Like glacial moraines on land, the grounding-zone wedges provided a record of glacier retreat and allowed Dowdeswell et al. to estimate that the rate of ice retreat at a resolution measured in days or weeks, revealing regional post-LGM deglaciation rates of 40-50 meters per day. "Perhaps most importantly, Dowdeswell et al. demonstrate the immense value of high-resolution seafloor mapping in unraveling the complex history of glacial dynamics. Only fractions of the seafloor in the hard-to-access ice-covered polar regions are mapped, and much is left to discover and learn," writes Martin Jakobsson in a related Perspective.