Theory: Trees on the island are touching trees outside of the island, and therefore preventing a cavein.
Could be. I didn't even think of that. I guess I'll see.
*EDIT* Game just locked up after I closed an airlock on my eastern gate. So it may have just realized 'oops, this is supposed to fall!' Hopefully no one was near it, since I had them gathering spent bolts.
*EDIT 2* Had to reload, but didn't lose much other than a closed gate and a couple designations. However, you were right. I trimmed trees around the edges until it sucked half my fort into the void. Success!
*Edit 3* My airlock is what is causing the lockup. I guess it's something to do with the bait animal tied up in the eastern entrance.
*Edit 4* I have trees growing in the floor built over the hole I created. Most of it is iron, as I have a slight abundance of it (after arming and armoring 90+ dwarves, plus a bunch of bins and other items out of iron, and using around 600 iron bars for this floor, I still have around 800 or so bars, plus magnetite still to smelt. And I just found tetrahedrite as well). I tried to build bridges, but I kept having random collapses until the floor was built, and they knocked down several bridges repeatedly. Got tired of rebuilding them.