Anybody played OP's world in Adventurer mode? I've never experienced HFS, so when I played this map, and dug into the cotton candy as soon as I could. After the fortress was wiped out by clowns, I started an adventurer, choosing "Human, Play Now!" (or Human outsider I think it's called now) to start at the last fortress. Instead of starting on ground level near the dorf's fort, I was on a 1-square ledge with the clown's fort to the left and a yawning abyss (too deep to see the bottom) to the right. The program's RAM usage spike up to 1.5M and my fps was something like 3 frames per MINUTE, the program crashed before I could take more than 2 steps (which took about 15 minutes). I started another new adventurer choosing a human from the town a ways north from the fortress, and the ground was all cracked and the game started with the "You have discovered an eerie cavern..vortices of purple light..glowing pits..yada,yada" message. Again my RAM got eaten and the game crashed before I could do anything. So it looks like that initial cave-in affected a VERY large part of the world around the embark (if the spire is the center of the cave-in).
I'm guessing there was a large section of the map with full cavern openness and no passage density (kinda like this: http://df.magmawiki.com/index.php/File:Open100Density00.jpg
) in all 3 cavern levels. Without enough natural rock pillars (and adamant spires) to hold it up, the weight of the surface caused it to crash through the empty caverns leaving the adamant spire as the only remnant of the caverns. Or maybe the magma sea in that area drained away for some reason, IDK.