Tunnel to Endless Mode

The Tunnel to Endless Mode is a tunnel found in Checkpoint 11 (Checkpoint 9 from Version 1.0.0 to 1.1.0) which goes 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 exists. 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 after the columns, except for in Mayhem Mode, where the end of the boss room holds the tunnel. (Note that the columns are replaced with even more obstacles.) It has a door outline on one end, and an Automatic Door on the other end. (See pictures 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 an upright position.The first 7 images are of Stage 4, the last one shows the Boss Stage.