In 2012 a user by the name of Jlun2 made a TAS of this game and in it was a way to get out of bounds by crouching in a tunnel. We've been trying to replicate it for a while and thought it maybe TAS only but I now have a way to do it. You hold down (crouch) for 22 frames exactly, then hold A + direction the tunnel is for 2 frames. You will now be standing slightly in the tunnel where you can now jump and clip below the floor. This can be replicated wherever there is a tunnel I think, I haven't tested everywhere. This all sounds impossible but it is definitely a feel based trick so the more you get it the more you get a feel for timing. Also made a vid for more help

Update: Clipping to the right direction has a longer wait time. When crouching, hold down slightly longer than normal then hit right and jump and you should get it. Doing this trick to the left is 22 frames of crouching, while doing this trick to the right is a bit more than 22. Hope that helps, 3-4 is a tad more forgiving now with resets