Play the level as normal until you are about to get the warthog. At this point, turn around and follow the road. Eventually, you should see a box, an arch, and a doorway blocking the way forward. To get behind it, climb on top of the box, and get onto the first of the four edges jutting out from the arch. Jump to the two crevices jutting out in the middle, and run quickly to the third one. It's easier than jumping. Now, jump to the fourth, and try to get up to the ledge right in front of you. To do this, go back as far as possible, run, jump, and when you reach the ledge, crouch. Now that you're up there, you should find weird water to your left, and a ruins that you can barely see on the ground in front of you.
You can get ontop of the building to which the arch was connected if you want. Climb up the pillars until you get to the top pillar, and jump on top of the building. From here, you can jump into the pit, or do some other crazy stuff. If you jump into the water, you'll be in a black space - you're really still there, but there aren't any underwater graphics. If you jump, you can see temporarily. Make your way around the building, and continue along as far as you can to the shoreline. Eventually, you should see a silver plate on the building between the two beaches. If you shoot it, you will see a water effect.
Submitted by: Thor