SignificanceWe present a record of glacier ice dynamics and ice melt rate at the boundary between grounded ice and ocean—or grounding line—of Petermann Glacier, a major outlet glacier in Northwest Greenland. The traditional view of grounding lines implemented in ice sheet models in charge of projecting sea level rise is that they not migrate during the tidal cycle and experiences no ice melt. Instead, the satellite record reveals kilometer-size grounding line migrations—or grounding zones—with preferential intrusions along preexisting subglacial channels. The highest melt rates of ice are recorded within the grounding zone. Vigorous ice-ocean interaction in kilometer-wide grounding zone will make projections of sea level rise from glaciers potentially double.