The Ark Vacation
From Halopedia, the Halo wiki
Start the level at the Rally Point Alpha, where the Forward unto Dawn drops off three Scorpion tanks . Play through on the tanks until you destroy the Scarab. After the walker is safely destroyed, grab a Grav Lift in the small structure and have your friends do the same if you are in Co-op. Then, grab a Type-25 Chopper or a Type-32 Ghost (Choppers are recommended for their better grip and stronger boost) and drive back to the building that you just came out of. You should look for a small camp site with a comm node and a supply case that held a Fuel Rod Gun. Face the crashed Scarab. Turn left and you should see a large dirt mound with some trees on it up against the wall. The Chopper's or Ghost's boost should get you up there. Crouch Jump up onto the wall. Turn left and walk up the wall. You should feel the invisible barriers pushing you off the wall. Keep walking forward while resisting the push. You should eventually slide down slightly and you can continue walking. You'll drop down onto the top of the building that you came out of. Keep walking into the invisible wall and eventually you'll go past it.
You can also get on top of the Cartographer by grabbing a Chopper and a Grav Lift. If you stand on top of the platform in the middle of the area where you fought the Scarab in and look towards the spire where you can get inside look to the right a bit until you see a cliff, go to the cliff and park the chopper against the wall. Make sure that the Chopper is facing the cartographer. Now you'll need to place the grav lift on the edge of the cliff and drive your chopper into it, make sure you boost in mid air or you might not make it. If that doesn't work, you may have chosen the wrong cliff. Once you are up there you can climb up the Cartographer using your chopper as a step and crouch jumping.
What You Can Do
If you look up, there is a bridge going across the walls. You can search around for the way up higher on the wall. Use the Grav Lifts to help you if you can't jump up high enough.
Once you're on the Cartographer you can climb up and see the other side of the Cartographer even if it's just very rocky. If the Scarab is still working, you can jump onto it or sniper everyone.
Also, If you continue to move, it is possible to see the backside of the spire, which is partially invisible, allowing you to walk across invisible floors, or even possibly get inside the building from behind.
Get a grav lift and get out of the playing area the normal way. If you continue to walk past the spire, and towards the way in which you have to walk to get past the invisible barriers in the beginning you will eventually reach the end. There will be a large ramp off the ground above the wall near you. Use a friend to jump up onto it and follow it all the way up. You will be at the end, and it should have some marks that resemble a weird face/helmet type thing. Jump on you allies head against the wall, and both of you jump as you throw the grav lift down, and you should be launched high enough to get over it. Keep walking into the area where there is nothing programmed and you will hit a loading screen, then another, afterward, you will not be able to move, shoot or melee, this is because you have been deleted off the map.
If you go to the ground on the right side of the Cartographer, and you look down into the bottom of the waterfall, you can see a random Phantom just sitting there. At this point, there is no known way to get down to the Phantom.