Understanding how much water is stored in the ground (or subsurface critical zone, CZ) is fundamental to managing mountain water and other natural resources. Yet, we know very little about how much water is stored in the CZ and how it becomes available to people and plants. Here we use a novel set of geophysical observations to gain new insights into mountain water storage, which we use to develop new terminologies and frameworks for understanding water storage in granitic mountains ranges across the western United States.