Which of the following best defines the unsaturated zone?

The unsaturated zone is best defined as the area above the water table where the soil or rock contains both air and water in its pore spaces. In this zone, the pore spaces are not completely filled with water, which allows for the presence of air. This is significant because it plays a critical role in the hydrologic cycle, affecting processes such as evaporation, infiltration, and the movement of contaminants.

In contrast, the other options describe different conditions of subsurface environments. A fully submerged layer of soil refers to a saturated zone, which is below the water table where all pores are filled with water. An area saturated with groundwater directly correlates to that zone as well. The mention of a zone where no soil exists does not accurately reflect the unsaturated zone, as this layer is characterized by soil or rock material although it may contain varying amounts of moisture and air. Therefore, the correct understanding of the unsaturated zone enhances knowledge of groundwater dynamics and environmental health.