Cairo Station vacation
From Halopedia, the Halo wiki
The colloquially-termed Cairo Station Vacation is a small programming oversight in the level Cairo Station of the video game Halo 2 that allows one to explore parts of the level previously thought to be inaccessible, or simply just scenery.
How To Get There
First play through Cairo Station until you reach the area with the two ranger elites waiting just outside the door to the outside of the station. Kill the two elites, and any other ones who may or may not be present depending on level of difficulty. Then find the geometric half-arch that leads onto the top of the station. Jump up on it and onto the top of the station. Then walk over to the area closest to In Amber Clad, and jump onto In Amber Clad. There!
What To Do
All sorts of fun can be had here. you can walk around the entire length of Cairo Station, see the other MAC bay, explore the other ship port, or explore In Amber Clad herself- you can even get inside her engines with a little patience and good jumping and timing.