In the room that appears under the bridge in the lighthouse area, that has appeared now that I have lowered all of the water, when I go I don't know how to uncover the secret.
The comments above are valid. This tuning fork question can be answered here: What do we do with the U-shaped tuning fork pillars?
Solving this puzzle lies in listening to the vibrations made on each side of the controller when you get close to the tuning fork.
The explicit answer is: LT, RT, RT, LT, LT, LT, RT, RT, RT, RT