An __________ is a geologic formation of permeable rock, sand or gravel that stores large amounts of water.

An aquifer is a geologic formation that is specifically designed to store and transmit large amounts of water. It consists of permeable materials, such as rock, sand, or gravel, which allow water to move through it easily. This property makes aquifers vital for groundwater supply, as they can recharge and accumulate significant reservoirs of water.

In contrast, a reservoir typically refers to an artificial or natural lake where water is stored, often for purposes such as irrigation, municipal water supply, or recreation. A spring is a natural water source where groundwater flows to the surface, usually due to pressure or geological conditions, and it does not store water in the same manner as an aquifer. A well is a human-made structure created to access groundwater from an aquifer; however, it does not store water itself. Thus, the defining features and functions of an aquifer distinguish it as the correct answer in this context.