In level 27, Charon Volatile Mine (last level of the game), behind the yellow door there is a large "room" with several invulnerability boosts and invisibility boosts. This room makes some kind of 8 shape with smart missiles in one of the loop and mega missiles in the other loop. I've been able to get the smart missiles (access from the part of the map where the blue key is), but I've not been able to get to the mega missiles. How can I get them, where is the access?
The access is in the section where the blue key is, in the roof of a dark corridor. From the level spawn point, take the first door to the left. Go down one step and take the door in right in front of you. Follow the very first corridor to the left. The secret door is in the roof of that dark corridor.