Tunnel to Endless Mode

The Tunnel to Endless Mode was a tunnel only on a checkpoint which headed to endless mode.

Checkpoint 9
Followed by the Spiral, there was the tunnel at the end of Checkpoint 9. It was replaced by an upside-down door in version 1.2.0 since Checkpoint 10 now existed. The Tunnel was featured with a door outline on both ends.

Stage 4 (Classic and Training Mode)
In the 4th stage of Checkpoint 11, the tunnel is featured, except for in mayhem mode, where the end of the boss room holds the tunnel. It has a door outline on one end, and the Automatic Door on the other end. (See below.)

End of the Boss (Mayhem Mode)
In the boss stage of Checkpoint 11 in Mayhem Mode, the tunnel can be seen after defeating the boss. However, the Boss stage doen't rotate, so you will see the tunnel normally, in a standing position.