So I load up my game, intending to do a bunch of stuff in Therapist. A friend stops by unexpectedly, so I leave the game on pause for several hours, and then later come back to it. I do all the stuff I was planning to do in Therapist, commit the changes, and press unpause. It's maybe a month after my 1st wave of migrants have arrived, and I'm still setting up my initial workshops down on z-5.
One of my dwarves goes out to the pond that my wagon had been resting on and starts channeling out a larger pool, hopefully so that I can get all that water-logged stuff. Naturally, he starts the wrong way and then freaks out about standing in water. Then my FPS drops to 4-6 and my announcement log starts getting spammed.
From my gamelog.txt:
*** STARTING NEW GAME ***
The weather has cleared.
** Loading Fortress **
Loaded region1, Xestsuosmlo (The Enchanted Dimensions) at coordinates (107,86)
Loaded the fortress Ozorcerol (Subtlelens), colonized by the group Kunfath Ishen (Catsacks of Passion) of the civilization Zastekkud (The Crystalline Pick).
Summer has arrived on the calendar.
Ast Velnil has become a chief medical dwarf.
You have struck pyrite!
Digging designation cancelled: damp stone located.
Ast Velnil, chief medical dwarf cancels Dig Channel: Dangerous terrain.
Ast Velnil, chief medical dwarf cancels Drink: Dangerous terrain.
Something has collapsed on the surface!
Horrifying screams come from the darkness below!
Bomrek Almėshbesmar, Bone Doctor cancels Spin Thread: Dangerous terrain.
Nil Avuzalis, Mason cancels Construct rock Throne: Dangerous terrain.
Zan Litasttishis, Carpenter cancels Make wooden Bucket: Dangerous terrain.
Rimtar Rakusttan, Craftsdwarf cancels Make rock Mug: Dangerous terrain.
Besmar Nishurist, expedition leader cancels Dig: Dangerous terrain.
The Emu's upper body takes the full force of the impact, bruising the fat!
The Emu is caught in a cloud of silty clay loam!
Apparently, something happened down at z-150 that cause the entire region to collapse.