The spirit & time room is located at Kami's Tower. It's a place that typically slows time in it's own world inside. 1 Earth Day would equal 1 Earth Year in the this room. The hyperbolic time chamber is just a large dome shaped building with no back wall to it. When you enter inside the room all you see is pure whiteness, which goes on forever. The saiyans and Piccolo particularly use this to train in the Cell Saga to become stronger before the the Cell Games start. Without the Spirt & Time Room, Cell would have probably defeated all the Z Warriors and would had taken over the world.

People that were in the HBTC:

-Goku
-Gohan
-Vegeta
-Trunks
-Piccolo