Secret Places (7)
There are four cylindrical structures like the one shown in the image, each of them counting as a secret place. They are all found in the large main hallway. The level is symmetrical, so if you find one you can figure out the location of the other three. The secret places are too high to reach without a jetpack. If you don't have one, just use your head... and maybe an enemy's.
Behind each of the alien bases (green and brown), there is a large area filled with eggs. There is also a very conspicuous wall that looks like it wants to eat you. Fulfill its desire by jumping into it. You will land in a secret corridor full of goodies. Once you've found one of these secrets, find the same general area in the opposite base.
Scan along the ceiling of the Queen's underwater nest. There are three air pockets that you can surface into. Only one of them harbours a secret place.
"Tag is cool": Found in the designer signature box (requires
DNCLIP, but using the map editor would be easier). This message is referring to DukeTag, which is a special Multiplayer mode exclusive to this level.
These combination switches are found in the overpass walkway above the center of the purple lava stream. The correct combination raises the slimy walls blocking the green alien base.
These combination switches can be found underwater beneath the green alien base (after activating a couple of aqua tank switches, which can only be reached from within the base). All four of them must be activated in order to open the wall and get the blue access card. Watch out for the two crushers while you're activating the switches.
These combination switches are also in the overpass walkway above the purple lava stream. They are on the opposite side of the same cylinder from before. The correct combination raises the slimy walls blocking the brown alien base. Due to the way the engine works, you can actually flip these switches before they are available simply by pressing on the walls.
These combination switches can be found underwater beneath the brown alien base (after activating a couple of aqua tank switches, which can only be reached from within the base). It was difficult to fit everything in the snapshot, but there are three switches. All three of them must be activated in order to open the wall and get the red access card. Swim through the broken gear teeth to avoid being maimed.