Question

What happens to the rainwater that seeps through the topsoil

Answer 1

It becomes groundwater. If enough collects underground, an underground lake or in some cases, even an underground river may form. In most cases however, it simply remains in the ground above the impermeable bedrock, where it is called the "water table" and can be drawn up by such things as pumps or wells. If a water table is left alone and no more water is added, it may very well sink until no more water can be extracted from it. Of course, if it is close enough to the surface of the ground, it may well evaporate.