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

An aquifer is a geologic formation that consists of permeable rock, sand, or gravel, which has the ability to store and transmit significant amounts of water. This makes aquifers crucial for providing water supplies for drinking, agriculture, and industrial use. The permeability of the materials in an aquifer allows water to flow easily through them, facilitating the movement and extraction of water from underground sources.

Reservoirs, while related to water storage, are typically artificial or natural lakes created by damming rivers or streams to store water. They differ from aquifers as they do not involve subsurface formations and are not primarily identified by the layering of rock or sand.

Springs occur when groundwater flows naturally to the surface, often emerging from aquifers, but they are the point of discharge rather than the storage formation itself.

Wells are structures created to access groundwater by drilling into an aquifer, but they are not the formations that store water; instead, they rely on the properties of aquifers to provide water.

Overall, the identification of an aquifer as the correct answer reflects the understanding of geological formations and their function in the hydrological cycle.