The pool actually extends quite a bit deeper, into the ground, almost thirty feet, straight down. At the bottom, there seems to be a very, very tiny planar rift, leading directly to the Astral plane, where it seems the magic is leaking through. An improbable, but not entirely unheard of, natural phenomenon. Sealing it should be a fairly simple task; even a mortal wizard would be able to accomplish it, given time to study it.