What is the name for underground water filling spaces in rocks and soil?

The correct term for underground water that fills the spaces in rocks and soil is "aquifers." Aquifers are geological formations that can store and transmit water, making them crucial sources of groundwater. They are typically made up of permeable materials like gravel, sand, or porous rock that allow water to flow through and be held in the spaces between particles. This stored groundwater can be accessed through wells and is essential for various uses, including agriculture, drinking water supply, and industry.

In contrast, the water table refers to the upper surface of the saturated zone in the ground, which indicates where the soil is fully saturated with water. Reservoirs are man-made structures designed to store surface water, typically created by damming rivers, rather than representing underground water storage. Surface water pertains to bodies of water like rivers, lakes, and streams that are found above ground, distinct from the underground storage function of aquifers. Thus, "aquifers" accurately describes the specific geological formations that hold underground water, setting them apart from these other terms.