This achievement is earned for finding the seventh secret and can be found on Checkpoint 31 or 32
It can also be found in this video, with a text description of its location below
[5:51] - Secret 7 of 13
After crashing through a wooden patch in the concrete ceiling, instead of going to the right, go to the very left of the area in your submarine. Use the charge to hit the pipes twice, then exit the submarine to make the jump to the new ledge and ladder. Climb to the top to discover the secret.
Note: You can also choose Checkpoint 32 and backtrack to this area.