Secret Places (8)
When you start the level, jump through the vent grate and go right. Follow the path until you find the security screen. To the left of the security screen is a switch. Press it and then run over to the other end of the room into the door that just opened. In this room is a hidden door between the two armor powerups (as shown in the image).
When you come across the first mirror in the level, look directly across from it for a crack in one of the walls. Blow it up and jump into the hole.
Directly across from the water fountain is an unblocked wall. Walk through it to find a secret place.
There's a secret door to the left of the "Danger - Radioactive Materials" door (the one that leads to the flashing red room).
There are hidden doors on opposite sides of the acid pool (two secret places). First hit the switch to line up all of the columns in the pool. Now go to the middle of each side and press on the walls to open them. One contains atomic health, and the other contains RPG ammo.
Continue through the level from the acid pool area until you get to the short elevator. After going up the elevator, walk down the open corridor and watch the left wall for a crack. Blow it up to find a jetpack.
After unlocking the blue access switch, the entire ventilation system becomes accessible. We are looking for a specific opening near the red access switch and sprite bridge. The quickest path to it is in the area beyond the red access switch, hidden in a corner. Crawl through the vent to the first available opening. When you step on the floor of this opening, a door sound plays and a tiny door opens on a nearby wall (look for the yellow close to the center of the image). You need a jetpack to get to that opening, or an enemy's head if you're clever.