Theory: Hazy Maze Cave is an Abandoned Mineshaft

Abstract
Hazy Maze Cave is an abandoned mineshaft, used to mine metals from the Metal Cave. The cave was closed off and abandoned due to a Scuttlebug infestation and dangerous gases.

Arguments

 * Argument 1: Metal scaffolding, work elevators and nets protecting walls are common all throughout the cave. Additionally, maps of the cave and many signs containing directions are also seen spread out across the whole level


 * Argument 2: The Metal Cave is a known location to whoever travelled through the caves, not a secret, as directions to is are written in various signs in the level. Many green ! Blocks can also be found in HMC, mainly in the Hazy Maze room, where the cap's use is recommended on signs warning about the toxic gas in the room.


 * Argument 3: The entrance to the Metal Cave is located inside Hazy Maze Cave, suggesting a connection between both rooms.


 * Argument 4: In the underground lake, a locked gate blocks the entrance to a room labelled "Abandonend Mine (Closed)", inside it, many Scuttlebugs can jump out from holes on the ground, suggesting that the rest of the path might have been closed due to them, as Scuttlebugs are the most common enemy in the level.


 * Argument 5: Additionally, the Hazy Maze room is dangerous and closed off, likely due to the dangers the "strange cloud" presents to the workers, even after attempts to circumvent those issues with the use of the Metal Cap.