In June 2024, a destructive flood impacted the village of La Bérarde in the French Alps. Rain, snowmelt, and the drainage of a surface lake on a glacier cannot fully explain the flood magnitude. We used glacier topography to estimate how much water could also have been stored beneath the glacier before the event. Our results show that large volumes of hidden water may have existed and could have amplified the flood, highlighting an overlooked hazard in debris-covered mountain glaciers.