Did I miss something? I thought cave-ins could happen if a room gets bigger than 5x5 with no support. I remembered this one time I was completely renovating a mountainside to turn into a veritable garden paradise, when a few of my dwarves decided to clear a cliffside before they took off the layer on top. They created a brittle crust of land upon which they then began to work upon and it broke. My whole screen was smoke and rock chunks. Bloody rock chunks. In honor of them, I flooded the area with a stagnant lake and used the land to produce strawberries.