Descent into the main shaft
Posted April 2nd 2006, renamed 2019
In the main facility, you can descend into the shaft that MC peers down into from the 'cutscene platform'. Way over on the far side, there are sets of prongs sticking out for a long way down. A Warthog-assisted jump gets you onto the prongs (from where you can launch a fun-filled prong attack on the enemy, incidentally), and from there you can progressively jump down. I'll elaborate below. In the plan I give here, you descend without ever going down to activate the Silent Cartographer, as there's no need for that (though it would give you extra enemies to attack from a prong).
Saddle up
You'll need a Warthog for the first jump. You could bring along some passengers too, for the amusement of hearing them scream. Have perfect health, and an undamaged overshield.
Get through the unlocked door
Drive the hog into the main facility and down to the door that you unlocked from the security substation. It doesn't look like you'll be able to get the hog through, but you can, as follows.
Drive through the doorway at an angle to at least get the front wheels through, then straighten up. The front wheels are now wedged in. There are two ways of getting the rest through. The quick way is to blast it with a few rockets. Alternatively, dismount and fetch another hog, and use the rear of that hog to ram into the first, forcing it through further until its back wheels are against the doorway. On a good day a single bash will do it. Then turn your hog around and ram forwards to force the first hog right into the tunnel. You couldn't use front ramming earlier, as it would've just tended to lift the first hog.
Squeeze back into the first hog. You can use a bit of crouching to get back in, but if you have trouble, try whacking the hog to move it along a bit first. Turn right towards the platform, then back up into what will be the starting position for your hog-assisted jump off the platform.
Trigger the cutscene
Get out and trigger the cutscene on the platform. You get a checkpoint after that, which is handy for reverting to if you kill yourself shortly. Incidentally, at this point you could spend a little time causing a few Warthog mishaps if you like, before getting on with the descent.
Reach the prongs
Now do a hog-assisted jump to get yourself over to the prongs. Hit the accelerator and curve around to the right a bit as you head onto the platform. Dismount as you're nearing the edge, or shortly after. Once free, you have a small degree of drift control which can help you land on a prong. Crouch as you land, to help absorb the impact and stop yourself bouncing back off. Your overshield allows you to survive with perfect health. You'll probably be on the third prong level down, of the seven; but landing healthy on the fourth is also viable. Thanks to the drift control, the assisted jump is actually quite easy once you've got the knack.
Another method is to stay in the hog, perhaps swerving as you leave to give you a stabilizing spin. With luck, you hit and bounce out onto a prong. You might even come to rest still in the hog. However, this method gives you no drift control, so it'll probably take more attempts before you get lucky, if you're interested.
Incidentally, you might get a checkpoint on your prong; that's occasionally happened to me, though I'm not sure quite why it was triggered.
Keep on jumping
Now you're on a prong, you can successively jump down to lower prongs. To help cushion these jumps, crouch as you land; and it can also help to slide against the wall to some extent. Actually, you can have quite a bit of fun with the first few jumps while your overshield is still in decent shape. You can leap 'crossways' from a prong to the one opposite on the next level down. MC the daredevil!
On the seventh prong down and still with perfect health, go to end of the prong and jump down to a surface with a house-shaped hole in the middle. Land on the sloping surface just past the flat section above the 'roof' of the house-hole. You can make it down with perfect health. Go down a slope to where this form joins the wall. On the left or right, jump down a level (slide against the wall to slow you up a bit), then another, then another. You're now trapped with walls all around you.
Actually, there's another way down off that holed surface, by jumping down near the 'floor' of the house-hole. I'll leave you to explore that.
A sorry end
Once you get down to your four-sided prison, there's not a whole lot you can do. Looks like you're gonna die here. What the heck were you thinking anyway, coming down here? You're supposed to be saving humanity! I spent a while writing "MC" on the walls with all the weapons I could get my hands on, just for posterity. Pistol, AR, plasma pistol, and finally the chain-gun of the Warthog that had landed here. It wasn't easy with the chain-gun because I couldn't see anything; my flashlight went off when I boarded the hog.