Tomb Raider Firestarter Challenge objective: Ignite six Corpse Bags in the Geothermal Caverns area.
Important note: You have a limited number of arrows, if you run out of arrow ammunition, return later by using Fast Travel.
In the huge cave, where cannibals kill a man after a short cutscene, the first Corpse Bag is hanging from the ceiling.
From the huge cave, chose the second tunnel to proceed further. When you reach the part where you should jump up to the higher level, look up, and you will see a Corpse Bag between the rock cliffs, just above supporting beam.
The third Corpse Bag is hanging next to the bulb in the tunnel no. 4.
The next Corpse bag is behind the door, adjacent to flammable gas.
After the second camp, proceed further and you will find the next Corpse bag in the second room.
When you reach the final zone of the cave, the last Corps bag is at the entrance of the room with the altar.