Level 13 - Unfriendly Skies (7 secrets)
1. GO into baggage claim, JUMP on the track, and JUMP off when you see a red button on the right. (Note: this thing is a press, a squishing thing made for you!). So, USE the button, then JUMP in and run into the open panel before...you know.
2. Go upstairs of "IMPULSE GIFTS". USE the cash register. A panel infront of you opens. Get cookie.
3. Room with 3 phones, 3 Pachinko machines, a black and a blue dispenser machine. JUMP on the Pachinko machines, then JUMP onto black dispenser machine and GO into open panel. Now you might say, "WHAT OPEN PANEL? THE PANEL IS CLOSED! YOU IDIOT!!", Oops! Sorry, forgot to mention you need to get the red pass key from inside the airplane cabin, USE it on the elevator door and call the lift by USING the button. Only after you've doing ALL that will the panel above the black dispenser machine open.
4. In control room, JUMP on blue "drink/soda" machine. JUMP west onto the dispenser machine, face East and SHOOT button. Get ripper heart (to the west).
5. GO to place with lots of Pachinko machines and an airplane cabin stuck inside it (next door to secret #3). Face West and SHOOT button on eastern side of the 3 Pachinko machines by themselves. JUMP on the machines, GO in open panel. Get missile launcher.
6. In room with plane's engine, JUMP and USE, Calligraphy (Japanese writing). Panel opens. Get rail gun.
7. GO to elevator door, the one with signs "GATE 21A" and "GATE 21B" above it. Then follow the following steps:
Push button so that lift arrives and the door opens.
Quickly PUSH/USE button outside lift again and then rush/run into lift, you should be standing on the top frame of the lift.
JUMP and USE button above. GO into shaft. Get nuke and **SECRET LEVEL** (Goes to Level 22 - Auto Maul)
Non-secret things to note:
In the room with the level end symbol, there are 2 "pillars" on either side with sloping ledges. JUMP on these ledges then JUMP NOT walk, JUMP against wall. Get nuke.
Level 22 - Auto Maul (6 secrets) **SECRET LEVEL**
1. In "Showroom", JUMP in water behind red car with one tree. Get clips, etc.
2. Near secret #1. USE PC, filing cabinet sinks down. Get caltrops, medikit.
3. Near red "mitsubishi" truck with 2 trees on either side, BLOW up crack. Get gas bomb, etc.
4. Next to secret #3, at the back of red "mitsubishi" truck, USE blue wall (facing West). Get missiles, smoke bomb, etc.
5. Place with garage door with no key required, 4 phones and a blue "drink/soda" dispenser, USE third phone counting from the left, blue "drink/soda" dispenser opens.
6. In repair workshop "office" NOT the workshop itself. Switch off the lights, yup...this is dumb..USE the light switch. Bookcase will sink. Get grenade launcher, etc.
Level 14 - Crude Oil (3 secrets)
1. Place with 2 PCs and mainframes. USE the "Shadow Warrior" PC. Mainframe next door sinks.
2. Inside "door with red pass key", GO into office, in corridor behind desk (or small sloping ramp) there is and airvent (facing South). Well you know the procedure...SHOOT..JUMP in..etc.
3. Office with "fish tank". USE PC monitor. Panel behind opens. Get rail gun.
Level 15 - Coolie Mines (5 secrets)
1. Place where blue key is found. To the East a crack will blowup by itself when you approach. GO into lava crack.
2. In corridor after "runaway" drill, BLOW up crack. Get armour, etc.
3. In Eastern corridor of outer "mini volcano" (the crack in the ground that erupts slightly upward) there is a small chamber where apparently lava is flowing from. USE it. Get cookie.
4. Drill that has crates near it and a control station infront of it. BLOW up the crack behind the drill and GO in and around to locked chamber. Get rail gun, etc.
5. After being blown (get it? Blown?) sky-high (literally) by fan, JUMP into other fan's "chimney" and BLOW up fan from the back of it to get back out.
(Source: http://www.pcgameworld.com/ Website)