After grabbing the guns, go back to the pool where you started. Get on the boat.
Once on it, shoot the rope.
Once the boat reaches the ledge, you will be able to jump and grab it. Don't worry. You will have enough time to grab on it.
Lateral jump.
Go up, and do a backwards jump.
Go around.
Another lateral jump.
And press SHIFT to drop down in the cave.
You've found another artifact!
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