The Dangai Cave is a secret cave residing within the Dangai Precipise World which has been used for training in the past. While in the cave, not only does time move differently from Soul Society and Human World, same with the rest of the Dangai, but those who enter in the cave also stop aging temporarily; both of these effects literally allows them to train for eternity and still be the same age when they emerge. It hasn't been used in over 1,000 years from the point the series has started, and the only one who's known to have used it was allegedly Captain Commander Yamamoto. The creatures that reside within this cave are known as Phantoms.