Step 1: Enable the eclipse by pressing all 4 buttons at the spawn. The eclipse must be reactivated everytime after every step.
Step 2: Find the temple next to the underground tunnel Just before you reach the PAP plate and look for a button on the temple on a statue. Press the button and hear the people that are trapped talk.
Step 3: Watch this video for further instruction on step 3. Fairly complicated.
Step 4: Go down the slide and listen to what the mystery characters are saying. Then after they stop, have your teamates (4 people) stand on the "pressure plate" until the eclipse ends. The pressure plate is the metal floor right after the slide. You should see another orb pop up right outside of the tunnel to the right of the temple.
Step 5: Go over to the waterfall and shoot the diamond thats on the ledge with a ray gun. When the diamond falls to the ground, make sure to shoot it with the new wunder weapon and knife it. The ball will automatically roll down the slide and up the water spout. A new orb will appear. The orbs are for knowing that you did it right.
Step 6: Go into the underground tunnel near the 2nd step and also near one of the PAP pressure plates. Find a wheel attached to a pipe and press X on it until the gas stops. The characters will say something about the walls closing in and too help, but thats all i figured out. Hope this helps.
*EDIT* Step 7: As you walk around, the gas leaks are mentioned by your characters. Have the napalm guy run around and blow up near the gas leaks.