In this tunnel, the colors of normal tiles are random. Moreover, the levels of this tunnel mainly consist of jumping along platforms, so most of the time Runner is the preferred character. There are also big chunks of crumbling tiles, which normally fall away once you step on one, because they are all linked.
For all of these levels, Child is also a preferred character due to his advantages in environments with large patches of crumbling tiles.
The following is a brief description of the story in the M-Tunnel. More information on the story can be found here.
After beating Level M-5, a cutscene will begin in which the Duplicator is watching the Runner's attempts to reach a tunnel in the distance. She kept trying, but she fell short every time. The Duplicator asks the Runner if she has gone insane, but the Runner denies it. When she respawns, the Runner notices that the Duplicator drew on her map. The Duplicator says that this is to "commemorate the moment" she has gone insane, before the Runner finally makes it past the gap and reaches the F-Tunnel.
|Not too easy|
Difficulty of the Levels of the M-Tunnel