Happening right now. The Mountainhomes Volalrėrith, "Whitechaos" is experiencing enormous amounts of cave-ins. The reason? A forest fire.
Just beneath the surface are indoor pastures with sheep. Directly one layer down: more sheep and kangaroos. Next: Giant Tiger pens. Then: A spacious corridor, the main entrance to the fortress. The next three levels belong to my central hall, with masterwork statues and everything. Next: Graves. Next: mausoleums for nobles and legendary warriors and other professions. All those collapsing surface falls all the way down, taking livestock, dwarves, masterworks with it, while spraying yellow sand everywhere.
The biggest lost may be the natural floor, because it can't be replaced, constructed floors cannot be engraved. But my defense is weakened as well: Almost all of the wooden cages in the traps overground burned down, my roof has holes in it.
The cause for all this: Cobim Ruluebbak Tul Luto, an ancient dragon, older than time (which makes her at least 432 years old). She appeared on the map, lit a small fire on the edge and walked right into one of my cage traps. I thought it was my lucky day, I finally caught a live dragon (the only previous dragon that attacked one of my fortresses (in DF2012) was only about hundred years old and unceremoniously stabbed to death by two human caravan guards (they didn't even take a scratch or a blister). Only that the small fire turned out to be a lot worse than I thought.
Right now the fire is moving towards the dragon, who is IN A WOODEN CAGE. Since dragons are fireproof, I suppose there will be a dragon outside of my base in the near future, and most of my cage traps are uncharched, so I would have to let her come into my entrance hall, where I have a few more traps.
So remeber: Only you can prevent forest fires. Unless a dragon comes around.