This trophy is earned for finding the tenth secret and can be found on Checkpoint 46
It can also be found in this video, with a text description of its location below
[10:02] - Secret 10 of 13
From the checkpoint, swimming to the right will reveal a yellow cable. You'll need to swim up, through the building and into the next area. Then swim down and to the left instead of continuing. You'll find a small vault door that you can open by rotating the middle piece and pulling the tab down. Follow the path to the secret from there.