There is a layer of stone on the surface of the earth. This is the bedrock. There are cracks in the bedrock, and the water is in the cracks, including the mountains. The place lower than the mountain is covered with clay, sand and pebbles. This is a loose layer. The clay has a small particle size, there is no gap between the particles, there is no water, the grain size of sand and pebbles is large, and the gap between the particles is large. Water is here. Thicker from 0 meters to 1000 meters may have a thicker place, I have not studied it.
The city is generally built on the impact plain, and it is also the key target of the water source heat pump. The groundwater in the impact plain area is often taken from the groundwater in the loose layer.