Jump on to the ledge to the right as you walk down the hall, and follow the instruction to shoot the wall section on the end. That reveals the first secret. Collect the shotgun shells and cross to the landing on the other side of the hallway. Run and jump across to the platform beyond. Run around to the right and grab the green armour. After getting the armour, return to the main area, enter the doorway and kill the grunt. Press the switch and the lift will descend, face right, and be ready for a rottweiler and another grunt, on the other side of the bridge. After killing them, drop into the water, head right, and keep going until you find another secret area containing a health boost. Get it and continue though the water, take a lift back up to where you grabbed the armour and return to the bridge. Cross the bridge and open the door, inside are several grunts and rottweillers. When shooting them try to hit the radioactive boxes to kill them for you. Make sure that you are not in the blast radius.
There are two ways of getting the next secret, the quad damage. The first is to walk the wall to the right hand side of the door. Look at the pillar and shoot the screen. The floor rises up, turn right, shoot the screen with Earth on it and walk in to the passage behind to grab the quad. Drop off the ledge and continue around the corner, a grunt and rottweiller will be waiting, kill them and take the life to the platform above. Shoot the waiting rottweiller and continue on the platform in to a room with a nailgun. As you collect it a wall rises revealing a grunt and a handy box of nails.
Exit the room and walk towards the slime pit. Kill the grunts on the other side of the pit, then use the button to create a path across. When you are halfway across, stop and aim for the red circle on the wall to your left. When you shoot it, a door opens to your left containing a double-barreled shotgun. Collect this secret and then continue across the pit. Be prepared for the grunts waiting for you around the corner. If you didn't pick up the quad damage earlier, shoot the differently coloured wall to your right to reveal an alternative passage to the quad. Continue down the ramp. Kill the enemy and hit the three buttons as you descend. The levels final secret area is in the slime. Behind the pillar is a biosuit. Grab it, then jump over the ledge into the slime.
Once in the slime, turn around and head for an opening, then following the passage around. In the distance is a ringed opening back above the slime. Jump out of this, grab any health you need and the yellow armour. Jump through the portal. Now head all the way back down the ramp to where you found the biosuit. A rottweiller and a another grunt are waiting around the corner, up the ramp from them are two more grunts. A wall section to the right can be shot to open to area where you found the second secret, the health boost. Exit though the slipgate for the second level.