Warning: I did this entire thing backwards. Also very minor spoilers.
So, I found my way to the Stony Creek Cave, and found a waterfall straight ahead. I managed to do a bit of wall-hopping to get up on top of it, and then headed into a large basin. After swimming around a bit, I found a spot in the floor where you can see through a rock - I swam through that and went straight down, out of the bottom of the world. After a few seconds of this, I dropped down into the Kagrenzel ruins, by the entrance. I walked into the ruins, found a button, and pressed it. It then plummeted me down into the same basin I had just fallen out of. Then I went down through the bottom of the world again (to make sure it wasn't just a one-time glitch), back to Kagrenzel, and out onto the world map.
Thought that was fun, wanted to share. So anyone who's found Kagrenzel or Stony Creek Cave should go fall through the world! :D