Gravity
From DwarfFortressWiki
Gravity vs. adventurers:
Gravity vs. human buildings:
Terrain collapses instantly, but creatures fall slowly, ie, not instantly. The exact rate of creature falling, whether they accelerate, and how damage is calculated is unknown. Interestingly, when a creature is sent flying by a hammer blow or similar over a cliff top, it will fly perfectly horizontally, as it would over ground, then fall straight down. Also, it appears that when constructed objects fall, they turn into their component materials and then fall 'slowly' just like creatures.
