The Hyperbolic Time Chamber, also know as the The Room of Spirit and Time, is a inter-dimensional room used as a plot device in the DBZ series. Spending a year in the room is equal to a day in Earth-time, so fighters can prepare for battle in a short amount of time. While this room was quickly introduced in Dragon Ball, it played a major role in Dragon Ball Z's Cell Saga where Gohan, Goku, Trunks, Vegeta, and Piccolo trained to defeat Cell. There is a limit of 48 earth-hours that one can stay in the chamber. If one were to stay longer, the door would vanish. Though, with the right amount of energy, one can make a portal back to Kami's Lookout. Majin Buu and Gotenks both did this during the Buu saga, after Piccolo destroyed the door from the inside.