Getting to the Red Key Slot (13)

In the Storage Room, hit the second switch to lower the second Forcefield.

Enter the resulting room and defeat the three Octabrains initially present, plus a fourth hiding behind a box. You can find a Holoduke, Shotgun Shells, Chaingun Cannon Ammo, and two Large Medikits among the boxes.

SECRET 4: Use the computer tower in this room to open a nearby crate containing Scuba Gear and Body Armor.

SECRET 5: Near the computer, destroy a crack in the wall. Inside, kill an Octabrain to get Pipebombs and three Small Medikits. Dive into the pool to find two Microwave Expander Ammo and see a broken UFO in the wall.

SECRET 6: Near the Pipebombs, activate a handprint on the wall to open a passage to a Teleport Pad. This pad leads to a bridge in the Outside portion of the level, which in turn leads to an RPG.

Return to the Storage Room and go down the small ramp to find a crane.

SECRET 7: Get picked up by the crane or use a Jetpack to reach the area with the conveyor belt. Here, you can find a Shrink Ray and a Jetpack. Activate the switch to deactivate the conveyor belt and open the door leading to a Teleport Pad at the belt's end.

Enter the Teleport Pad and emerge in a cramped area with a switch. Activate it to enter the UFO (this is the secret). Inside, you'll find an Atomic Health.

Proceed past the conveyor belts in the UFO Room, where you will be attacked by three Alien Drones and a fourth hiding in a corner.

Collect a Pistol, two Chaingun Cannon Ammo, and four Small Medikits found around the room.

On top of the ramp leading to the Yellow Key door, find two Devastator Ammo.

The Red Key door is also located in this room; enter it.