Why is there water under the ground?

Answer 1

After it rains, water does one of the following:

1. Flows along Earth's surface into a river or reservoir
2. Sinks into the soil (caused by gravity)
3. Evaporates

Some of the water that sinks into the soil is used by plants, but the remainder of it sinks even deeper into Earth. The water eventually reaches rock, an impermeable level of Earth, and can't sink any further. It then becomes groundwater.

Source: U.S. Geological Survey

Answer 2

Rain water and other ground water seeps into the ground and gravity holds it there. There is space between rocks and sand particles, and water finds its way there and stays if nothing makes it move. Sometimes there is a pump or a lake or some other place for the underground water to flow and it will go that way. Imaine that you have a large pool of water. Add sand and gravel to the pool and eventually you'll put in enough that the water will be covered and you won't be able to tell the water is down there.