The geologist had tracked the glacier for thirty years. Each decade it crept one row farther south, a white wall consuming the valley. But when it reached the third row, something strange happened: the center three cells of the advancing front began to retreat. Satellite thermal showed a volcanic vent beneath the valley floor, its heat carving a concavity in the ice. The edges of that row still advanced, but the center was hollow. By the next survey, the cold had won and the front was solid again.