Caldera

The caldera was a location on the Homeworld that acted as a node point where all lines of historical influence converge and one can stare directly into the heart of time. It was created during the initial invasion of the Yssgaroth during the anchoring of the thread. (PROSE: The Book of the War)

In tapestries on the Homeworld, the caldera was depicted as a black disk with Yssgaroth swarming out. (PROSE: Against Nature)

Behind the scenes

 * The caldera is described as looking very similar to the Eye of Harmony.
 * The caldera is also very similar to the Untempered Schism, later seen in several Tenth Doctor television stories and comics.