A fissure in a secluded location of a mountainous area of Gallifrey from which the chronoplasm substance wells up to the surface. The pool developed as a byproduct of the combination of refuse used in temporal experiments by the Time Lords and hyper-dimensional energy from a time/space fissure created by those experiments. When originally discovered by the Time Lords, it was deemed forbidden and highly guarded, but over millennia, its location was eventually forgotten.