I stepped through the door and was underwater!
Then I went into the GECK and saw that my land was thousands of meters below the water level. I put land 1000 meters above water, then loaded it up again. I was still underwater.
I looked at it again in the GECK, and chunks were missing. I started it up again in-game, and the world was wrapped around itself, walking from one street to the next teleported me sometimes underwater, sometimes next to a mountain I don't remember placing!
Whee!
Any thoughts?