Final answer:
The layer underground where all empty spaces are filled with a combination of air and water is the zone of saturation.
Step-by-step explanation:
The layer underground where all empty spaces are filled with a combination of air and water is called the zone of saturation. In this zone, the soil or rock is completely saturated with water. Above the zone of saturation, there is a layer called the water table, which marks the top of the saturated zone. Below the zone of saturation, there is a layer of soil or rock that is not completely saturated, known as the zone of aeration.
Learn more about underground layers