Back in like 2010 or 2011 there was a way to glitch on top of a cliff to the right of swamp town, if you walked to the right you could spot 1 tile being different from all others. If you walked up on that exact tile you got warped into a cave level (was pretty big as it was existing of multiple levels).
The reason for this was is that they most likely changed the tiles but left the npc scripts in the level.