Here's why:

* Saturated Zone: This is the area below the Earth's surface where all the pores and spaces in the soil and rock are completely filled with water.

* Unsaturated Zone: This is the area above the saturated zone where the pores and spaces in the soil and rock are only partially filled with water, with air also present.

Groundwater is the water that fills the spaces within the saturated zone. It is a valuable resource for drinking water, agriculture, and industry.