Jump in the moat. There's a underwater cave with some stuff.

There is some green goo, and a high-voltage discharge into it. High up on the insulator is a switch. Shoot it, it opens a small secret door.

This is the squasher. You need to start it. If you get caught in front of it, shoot the button beyond the axes. Get behind the squasher (There is sufficient room behind it to survive), press the key on the back wall. It will open a secret room between the two blue-key doors.

Darn cheap wooden floor. To get away from the charming, but rather waterlogged zombies, you need to run to the end of the two corridors marked with red arrows, and throw the switches. That opens the bars in the left side of the pic.

There is two candles in each corridor. Shoot all four of them, a secret door opens opposite the exit.

After you finish stumbling around in the swamp, you take a lift up to a nice, dry room. On the upper left-hand side, there is a red switch. Get up to the wooden thing above you. Secret. Mr. Teddy Bear in the next room may cause some trouble in the process.