In this save, it would appear the entire world collapses on itself, and the glowing pits are revealed. This happens after unpausing, it takes quite a while though, and then you get 1 fps. Occasionally I get messages that a section of the cavern has collapsed! And it would appear that my dwarves are falling. After a long enough time, you start getting messages that things are exploding after colliding with an obstacle. Confirming that the entire world has collapsed out from under the dwarves in question.