Part 4: Secret Level 01
As the level name implies, Segment City is divided up into segments. Each segment is separated by a one way door. A couple of these doors are also accompanied by grating that will appear behind you after you fly past a specific spot in a tunnel, forcing you to either exit the mine or use the door.
There are two ways to circumvent this. One is to place a marker where the door is so that it cannot close. It's unnecessary for now, but it will become a common tactic in later secret levels in order to make some of the other trickery I need to do easier. The other way to get back to the beginning of the mine is through a secret door in the third section of the mine. Where the cloaked diamond claws are, there is a hidden door that will take you to the room above the tunnel to the reactor. It can be opened any number of times, so you can explore the mine again if you think you missed something.