Underground pools can be found buried in mountains. They are about the size of a magma chamber but only two or three levels deep. This is a finite water source. If the water is drained or pumped out, it will not be replaced.
Finding an underground pool (or an underground river) is required for farming tower-caps for wood, but that water need not provide the irrigation for those tower-caps.
Underground pools are also inhabited by various dangerous creatures such as frogmen, lizardmen, snakemen, olmmen, cave crocodiles, giant toads, giant olms, and mud men.