At the Feeding Area where you first encounter the Riepers, in the very expansive cavern look into the second grotto the little Rieper comes out of. Stick a mining charge, throw a grenade, and launch a rocket. These should at least partly reveal a secret passage. Follow it. After the loading screen, you will now be with "The Artifact". Go onto the piece of land supported in the middle of the cave and stand there a few seconds. Eventually, a message will read "Picked up fuel canister". Rarely this will actually be a fuel canister for the flame thrower, but it will usually be a Fusion Rocket Launcher. This is useful for Capek, but you can save it for anything else you need to vaporize.