Level 01: Waterloo Waterpark
There are really only two secrets on the first level. However, a third secret exists but it does not lead to weapons, ammunition or glory - thus, finding it is a personal prerogative.
The primary secret is the location of the shotgun. A secret door is hidden in what has come to be called the SHADOW ROOM. As finding the shotgun is such a vital step it has been included in the main guide.
The next secret is a lot more difficult to find. Sidestepping is
necessary and only the most accomplished players will be able to find
it. After turning right from the first terminal there's a room to
drop down into. This room is bounded by four raised pools. Stand on
the ledge and look at the pool at the top left - it has a platform
hidden within. On the far wall opposite there's a secret door. The
catch is to get up into the pool and onto the platform. Once on the
platform the secret door can be opened and a passage way into the
SHADOW ROOM can be explored.