Has anyone done this yet?? (hope not....*sweat*) Anyway, during the warthog "race" when you are at the part where Cortana tells you there is a big gap ahead, there is that big drop right? Well, if you adjust your aim left or right just a bit before you jump you can land on a platform. From there you can jump down and look at the "rex wall". But wait..there's more! Climb back to the warthog via the burning barrels and continue to the left (or right, depending on which side of the platform you land on). If you go farther you will see a ramp leading down to the wall. Use your warthog to ride down the ramp. Now from here you can get all the way down to the bottom of the pit.
I don't wanna make this too long...so if anyone is interested I will post a new message describing how to get to the bottom with extreme detail.