Start by shooting the switch so the green side faces out, causing the huge gear on the right to rotate clockwise.
Jump onto the block in the pool and use your torch to light one of the gas vents on the gear. The gear will then turn, carrying the fire to the gas vent in the right corner.
When it ignites, a golden ring is revealed above. Grapple the ring and climb onto the ledge above the gears.
Raise the Staff of Osiris as you push the time bomb into the gap on the left side of the grapple ring.
Set a remote bomb behind the time bomb...
...and detonate it to fling the bomb down onto the ledge with the switch. Then jump down after it.
Roll the bomb to the left side of the platform, opposite the ledge across the water. Then use another remote bomb...
...to hurl the time bomb over to that ledge.
Jump over there or wade through the water and climb out on the left side of the ledge. (The right side is a little glitchy and sometimes won't let you climb out). Then jump on top of the bomb to reach the final red skull. (5/5)
In co-op mode you should still light the first gas vent as shown above, but there is no time bomb. Instead one character can jump on another's shield to reach the red skull.
If you have found all five skulls, you'll receive the WEAKENED COPPER RING OF REACH (Bomb Radius+, Speed+, Weapon Damage-).
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