Gully

Gully is a room in the Tallon Overworld region of Metroid Prime.

Secret World

On the top level of the room there's a rock which you can ghetto jump off of to reach the highest portion of the room, where you can then jump out of the level from. Jump on this rock and slowly slide down until you know exactly where you'll fall off, then stop a hair short of that. Turn slightly so you're facing the highest point of the rock, run and jump twice. You should land on the highest ledge, back down a bit then run and do another ghetto jump over the clipping and out of the level.
This was secret world #12 in the original numbering scheme.