The underground rock layer which water flows through is called an aquifer.

Here's a breakdown:

* Aquifer: A layer of rock or sediment that is permeable (allows water to flow through it) and saturated with groundwater.

* Groundwater: Water that fills the spaces between rock and soil particles underground.

Aquifers are crucial for supplying water to wells, springs, and even rivers. They are important sources of drinking water for many communities around the world.