
This entire room here is built of a tower and has no support below it.
Directly to the left of the yellow cross is the offending tile.
Shortly before this screenshot there was a wall there. Because I tend to have problems with the outside corners of walls not being built so I had this wall built using access from the left (notice the floor tile). Once the wall was built I deconstructed the floor and accidently collapsed the wall.
I quickly rebuilt the floor and planned to rebuild the wall when I noticed the blue hole.

One Z-level below I notice that there must be water in the hole.

Another Z-level and we reach the farms, the tile to the right of the hole has been deconstructed and there is a unconscious cat showing no other injury (might be a coincidence). Also note the wells.

wahay
the yellow cross now is where the hole ends. This cistern feeds directly into the river, no floodgates at all. Until recently at the same spot as this mystery hole there was a door. Locked and tightly shut. The door disappeared and now there is about to commence a significant amount of !!FUN!!.
There stems my first question, has anything like this happened before?
The second is: How can my fort be saved?
The hole stops by now (I kept the yellow cross in the same place for reference). Nothing much on this level. Notice (turquoise X) a stunned miner at the bottom of the cistern.
Next is just the stairway (2x2)
Comes out into my slightly flooded workshop level containing all industries.
Food,kitchen,still (butcher and fisheries are above ground).
Never quite finished the dining hall...
Quarters level. Most of the 43 residents sleep rough. To the top left of the stairway is a small hospital. Only two injured people and both are physically able( though they might just rest through the flood till they drown).
After 3 levels of 2x2 stairs the caverns appear.
Another 20 levels the staircase narrows to 1x1.
46 more levels goes to the roof of the third cavern.
Using Phoebus tileset and Masterwork Fortress Mod while playing as Orcs.
So can collapses cause craters and does anyone know how to stop a fortress flooding?