On millennial or even centennial time scales, the activity of rapid flowing ice can affect climate variability and global sea level through release of meltwater into the ocean and positive feedback loops to the climate system. At the surge-type glacier Brúarjökull, an outlet of the Vatnajökull ice cap, eastern Iceland, extremely rapid ice flow was sustained by overpressurized water causing decoupling beneath a thick sediment sequence that was coupled to the glacier. This newly discovered mechanism has far reaching consequences for our understanding of fast-flowing ice and its integration with sediment discharge and meltwater release.