An almost invisible crowlspace.
This secret seems ultimately lost if you jump from the second rope to the platform on the right instead of getting the crowlspace at left (no return to the moved "box" with level from which you can jump to the first rope).
Observe on the floor the Levelgate to Temple of Poseidon. Tip: to get the second rope, align Lara somewhat at its left: the ceiling is sloped and the first rope tend to rotate to right.