From Halopedia, the Halo wiki
The level was originally going to feature a trench run that involved massive, horizontal grav-lift transporters that you could drive a warthog into and through. The warthog run took place in several tubes which were divided by the aforementioned grav-lifts, and John-117 was going to travel along these tubes in a warthog in order to board the Prophet of Truth's Forerunner dreadnought.
The trench run would've been timed, much like the trench run from the level The Maw in Halo: Combat Evolved. The tubes would've centered around the Conduit and many enemies would have fought along the geometry. The enemies that would have appeared in the run are the Flood, Jiralhanae, Unggoy, Kig-Yar, Yanme'e, and several Brute-manned Type-46 Spectres.
Some incomplete map geometry that was intended for the warthog trench run was left on the disk. The Halo 2 mod "Induction" re-used the map geometry in a multiplayer level.