Found a path in World 1 Zone 1 that allows the player under the world without any sort of actual bug or glitch. Video here shows in full detail how I got there just by jumping to ledges.
I’ve since heard of players being able to get under the world through an actual glitch in the same zone, by way of some water spouts behind the waterfall (I believe it was). If positioned right, you can apparently stand under the second spout and manage to get it to drop you under the world. I’ve not tried this method though.
Path under the world