The first secret level of Obsidian contains several puzzles, especially beyond the yellow doors. There is no yellow key in Argon Entrance, but there is in Level 4, near the second teleporter to this secret level. Outside the yellow area, there is a complex trigger system similar to that of Secret Level 2 of Descent II: Counterstrike.
This level contains a combination of puzzles, powerups and several triggers. Obtain as many goodies as you can.
You will only have access to the yellow area from Level 4, as there is no yellow key in Level 1.