Secret Places (4)
Outside the studios, find your way to the ledge of the enormous "Lunar Apocalypse" movie poster. Follow it along until you get to the triangular 3D Realms logo. You can walk through this part of the poster to find a secret place.
Still outside the studios, find the two cash registers. Push the cash register on the right. Now be quick about it and run to the snack machines around the left-hand bend. If you arrive in time, you can find the secret place behind the snack machines.
In the area just outside of stage 17A, there is a hidden door in the large stack of crates. First you have to climb to the top of the pile to trigger a touchplate. After the door is triggered once, you can freely push on it to open it. Use the spotlights, nearby crates, or an enemy's head to reach the secret area.
In stage 17A, jump into the Earth wallpaper to find a secret place.
"Push the register" (pause) "Not the snacks": In the room with the snack machines (nightvision goggles required). This refers to one of the cash registers outside the movie studios, as described in the secret place listing above.
"Jump at the Earth": In the spaceship movie set of stage 17A (nightvision goggles required). Appears clearer from a distance. This message refers to the Earth wallpaper found in the same movie set, as described in the secret place listing above.
Press on the red "A" (of the "USA" sign) in the Lunar movie set of stage 17B. A secret door will rise (and this door apparently has some problems, it rises up even further after a short delay). This secret nuke button warps to Tier Drops.