One of the pipes in this big room has a crack. Shoot to enlarge. But, before you jump in...

...Turn 180 degrees and fire a shot against the flickering white light. And you already have the next secret.

After you entered the pipe trough secret #1, you get flushed through the pipes and end up in a small pool. Shoot the two grids, and the secret will show at the 'X'. Watch your back, when you go for it!