-Hyperbolic time chamber-
The Hyperbolic time chamber is a place where you can train what is special about this place is that you can train for a year in there and it will only equal a day in normal time.You are only allowed in 2 which is 2 years.
The only people to enter this place are:
Goku:1yr 1month, Gohan 1yr, Goten 6 months , Trunks 1yr,
Chibi Trunks 6 months, Vegeta 1yr, Piccolo 1yr, Buu 3days