Despite the critical role of water in alpine regions, its presence in bedrock is frequently neglected. This research examines the dynamics of water in fractures using 1 decade of measurements from a tunnel 50 m underground. We provide new insights into alpine groundwater dynamics, revealing that up to 800 L d^-1 can flow in one fracture during extreme events. These quantities can saturate the fractures, enhance hydraulic conductivity, and generate pressures that destabilize slopes.