Move forward cautiously to eavesdrop on the prayer meeting taking place in the next cave. The sentry ahead is watching the ceremony so his back is toward you. If you sneak up on him quietly...

..., you can take him out with a Melee attack. Wait for the icon to appear on screen, then press the Melee button...

...to bash the guy's head in. He must be a habitual whiner since the other men take no notice of his moaning when Lara gives him the axe.

You can then loot his body and shoot the final sack (6/6) for the Firestarter challenge, which is hanging from the ceiling here. This doesn't seem to attract the any unwanted attention. If you've found and burned all the other sacks, you'll receive a 150 XP bonus.

If you flub the stealth attack, the 8 enemies below will be alerted to your presence. You'll have to fight them or reload the last checkpoint and try again.

If you succeed in taking down the sentry quietly, continue sneaking around to the right until you can see the man leading the prayer. Shoot a fire arrow into the billowing gas behind him.

This sets off an explosion killing everyone in the circle, including the demolitions man on the stairs on the far side of the room. Now that wasn't so bad, was it?

Drop down and pick the 2 edible plants growing at the base of the ledge where you were just standing. Loot the bodies and go after the last few collectibles in this area.

There's a bunch of arrows leaning against the back of the small wooden wall.

Lying on the ground near the altar is the last document (3/3), the final installment in the Diaries of a Madman. Pick it up. Then wade through the partly flooded tunnel just to the left of the altar.

NOTE: If you didn't explode the gas vent earlier to kill the Solarii, the tunnel near the altar will be blocked by rubble. Stand at a safe distance and shoot the gas with a fire arrow to blow open the passageway. (Thanks to Janet and Sraar for pointing this out.)

At the end is a rock ledge with a salvage crate and a relic (3/3), an Inscribed Silk Fan.

Return to the main cave and go up the steps on the right (southwest). Grab more arrows if you have room for them.

Then go around the gas valve into the little alcove on the left, where you'll find a food cache and the last GPS cache (5/5) for this area.

NOTE: If you earn a Skill Point and want to spend it before finishing this level, you must do so before opening the exit from this cave. Opening the gate sets off a series of natural gas explosions that prevents you from backtracking through the tunnels to the Catacombs Base Camp. So go there first, make your upgrades, return here, and then open the gate. You will still be able to return to the Geothermal Caverns via Fast Travel later on. By then the explosions will have subsided.