Room of Spirit and Time or Hypobolic Time Chamber

The Room of Spirit and Time is a room that is located in Kami's Lookout. The room is first introduced in the Piccolo Daimo Saga, when Goku trained with Kami in the anime, but, in the manga, the room was introduced during the Cell Game, as a room to train to get stronger. One day on Earth is equivalent to one year in the room. That is why the room is there. So, you can train for one day, when you could have trained for a year. Why haven't they just trained and trained in the room you ask? It's simple. Anyone that enters that room more than twice will be locked in nothingness for all eternity. The people that have entered that room that we have seen are:

Goku - Twice
Piccolo - Twice
Vegeta - Twice
Gohan - Once
Mirai no Trunks - Twice
Goten - Once
Chibi no Trunks - Once
Super Buu - Once
Krillin - Once
(Fusion) Gotenks - Once