This trophy is earned for finding the eleventh secret and can be found on Checkpoint 48
It can also be found in this video, with a text description of its location below
[11:31] - Secret 11 of 13
From the checkpoint, you will come to a puzzle where you rotate a platform at the end of a long piece of wood that is used to shield yourself from the light scanner. Position this at the very center at its highest position. Now go back to the left, and swim downwards and follow the path. You'll find a platform in the pool of water with the secret on top.