The Ark Vacation

The Ark Vacation is a glitch that can be performed on the Halo 3 Campaign level, The Ark, that allows you to bypass the normal map boundaries. It is a pretty cool glitch in my experience, but there are some gray areas. Disclaimer: I am not liable for any damage done to your Xbox 360 or Halo 3 disc for attempting this glitch.

Tutorial
To perform this vacation start a Co-op game, and get to the area with the Scarab battle. Get into the building with the Grav-Lifts, and get one, and get a Chopper. Once you have a Grav-Lift and a Chopper, make sure you have triggered a checkpoint, or else an invisible wall won't disappear and you won't be able to do this glitch. Then, go back up to where the tanks and Warthogs came out of before. Go to the right and boost up the rocks. Keep boosting to get past the invisible walls. When you reach the top, you will hit an invisible wall that can't be boosted through. Drive along it diagonally and you will slip past it and out of the map. Next, grenade jump up onto the pillars. Walk along them until you can see a little 2D line next to one of them. Jump onto this and if you die, get your Grav-Lift back. Walk along this until you get to the end.

Tri-Gates
There are 11 known Tri-Gates outside this map, all discovered by General Heed. To get to them, follow the steps above, and then walk to the end of the platform, and VERY carefully, crouch, and then jump off the edge on the right, and attempt to land on the dark ledge. From there, walk all the way to the end of the platform, until you get to the beginning of the long ass beam that goes up to the top of the map. Grenade-Crouch Jump combo onto that beam and walk all the way up (gonna take a while). Once up top, look in front of you. There is the first Tri-Gate. Use the Grav-Lift, and have the other person use their Grav-Lift on top of yours. You should now both be up there. The floor of that area is an overlay of the Tri-Gate. That makes 5. To see the other 5, carefully jump onto the invisible floor that is on top of the beams next to you, and be careful not to trigger a loading point. Walk all the way across and Butterfly Jump up there. The Tri-Gates are in the same spots on this side. There is also a small one on the Cartographer building.