From The TimeSplitters Wiki

Waterfall Glitch
Requirements	- Shrink - Training Ground- Small Character
Finder	The Glitch Finder

In TimeSplitters 2, there is a way to go behind the waterfall in the Training Ground map. To do it, first select Shrink from the Arcade Custom game mode menu. When prompted to choose your character, choose a small one, like Monkey or Robofish. When the match begins, let yourself be killed a few times so your rank goes down and you shrink. When you're near last place, go down to the waterfall. You should be able to walk right through the back of it into a glitched area with only sky! If you can't go through, get killed a couple more times to lower your rank. When you are in this glitched sky area, you can go out into the sky forever. Also, characters on the other side of the waterfall will attempt to shoot you but not be able to. You can back out just a little bit to kill anyone, but if you go out too far, you'll have to turn around and go back in.

NOTE: I believe this problem was corrected in TSFP's Training Ground map.