How does groundwater primarily exist under the soil surface?

Groundwater primarily exists under the soil surface in a saturated zone, which is an area where all the voids in the soil and rocks are filled with water. This concept is crucial in understanding aquifers and how groundwater systems function. The saturated zone lies beneath the unsaturated zone, where soil moisture can vary.

In a saturated zone, the water not only fills the spaces in soil but also in rock formations, providing a significant reservoir of freshwater that can be tapped into through wells. It plays a critical role in the hydrological cycle, maintaining stream flow during dry periods, and contributing to the overall water supply for various uses.

The other options do not accurately describe the nature of groundwater. Groundwater does exist in a liquid state and is not limited to being vapor; however, it is primarily found in the saturated zone, making it the most appropriate answer. Additionally, while groundwater levels can change with rainfall, it exists continuously and is not solely dependent on heavy rainfall for its presence.