Something really odd has just happened. I was testing out a custom creature that's supposed to temporarily transform into another creature under new moons (as opposed to full moons, like were-beasts), and tried to wait after transforming to see if I had set it to end properly.
Instead of zooming back to my creature after waiting, I was given control of a pond-grabber in the caverns. After accidentally air-drowning myself, i used DFusion to take control of a nearby troll, who was likely pointing and laughing at my dried-out carcass.
After acquiring some food from a nearby giant bat I used DFHack to conjure myself a pick-ax, and carved my way upwards to freedom. though it turns out that it will be a bit harder than that. When I broke through the final layer of rock between me and the surface, I found myself under the ocean. On the exact opposite end of the map from where my custom creature was.
It turns out that the ocean is not an even surface of water. Some regions have their water level higher than others, and I occasionally have to swim upwards as I'm engulfed by tidal waves.
As I'm typing this, my troll is swimming his way to freedom, with his pick in one-hand and a pile of bat-meat in the other. Let's see how far he'll go.
E: Made it to shore. Tried fastravelling. Became Giant Toad on second cavern layer on the far northern part of the map. I quit.