Hyperbolic Time Chamber
The Hyperbolic Time Chamber was an empty pocket dimension contained within Kami's Lookout, where time moves much faster (a year goes by for every day that passes outside the Chamber) and conditions are extreme, with area of heavy gravity, extreme cold, and burning flames. As such, it served as the perfect "quick" training ground for the Z Warriors during the advent of Cell.
The Chamber was breached by Majin Buu several years later, and destroyed along with the Lookout, but it may have been restored when the Lookout was reconstructed.
Known as the Room of Spirit and Time in the original Japanese.