Where does the water come from when you dig down far enough into the ground and why is it stored so far down below?

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.