It's a door.
Checking the door.
Mysterious Place is the 20th track in the Undertale Soundtrack. This somewhat elusive, ominous, repetitive, and atmospheric song plays at the Mysterious Door in Snowdin Forest. It shares instrumentation with Quiet Water.
- The version heard in-game is slower than the OST version. This is due to the game playing
mus_mysteryat 95% speed (as it were stored at 20/19, or 1.(052631578947368421)x, 5.(263157894736842105)% increased speed). This could be an error in the OST release or a bug in the game (v1.06).
- The duration difference between the OST version and
mus_mysteryis just trailing silence in the OST version.
- game version 1.06 and Undertale soundtrack downloaded on 2017-11-03