Shoot the big guy, go outside to the right. At the end of the wall is the 'secret'.

In cell block 'A', the second cell from left holds a secret. Shoot the middle bottom stone, duck and go for it.

You won't like this one. The secret is inside the big lava pot - the same one where some unfortunate caged guy ends like stir-fry. A 'invunerabillity' might help.