You're now in a swamp. Pretty cool looking. You're looking for Keyes. First you find the downed ship and can hear their distress call. You go past some enemies and have to jump up a hill to get over it. You find the bad guys under attack by someone! Strange. You run right to get indoors.

Get on the lift and activate it from ON the lift. You'll go through a series of rooms with hardly any enemies. One hasno obvious exit (the one with turrets) - jump over to the ledge to get to a partially hidden door. You'll meet an insane guy. In that room you have to jump over some fire to move on. The next room has an energy bridge, with health to the left. Head down the ramp.

You see a little movie now - you hear a noise, find a dead marine, then find a helmet with Jenkins' story. In it you learn about the little bugs that attacked the marines.

Master Halo Walkthrough