I just set up a base camp on a level that has an aligned altar and a fountain which I've dug to make pools for dipping, and it struck me how convenient that is. Made me wonder if the fountain was a level up, could I dig holes around the room prior to digging the fountain (or over/adjacent to an existing pool) to create a pool in a level below? The wiki says that objects and monsters will fall through, and I've seen pools form in other rooms through open doorways (even quite far away from the fountain. Or does water follow a different protocol? Just curious.
After more than 50 attempts at filling rooms around a fountain with holes and creating pools, I conclude that the water does not fall through (at least if there are any undug spaces within a six space radius-eg, paths). Thorough searching of the next three levels down were always dry. Later, if possible/convenient, I plan to try this test using Levitation to completely fill the rooms, but I doubt it will be as thorough a test given the probabilities of obtaining the necessary item(s). I also doubt that the outcome will differ. I'll keep you posted