More info coming soon.
The actual name for this location is Dusting Fields.
On the top left hand corner of the dusting fields there is an entrance to the Mysterious Mine. You can see a strange shadow dancing on top of a stump above the entrance and a rock to the top left of it. Walk diagonally into the rock by holding right and up and you should clip through and be able to float above the map, but not be able to do anything. Strange glitched enemies that appear to be prototypes with inverted colors appear as various other enemies. If you walk across the world you can get to chests that you shouldn't be able to get and the enemies will change to bounce pots mixed with other enemies
To exit this stage, go back to the rock and walk down through it.