|This article is about an older version of DF.|
Deep pits are a type of hollow underground feature, basically holes of a fixed shape reaching down from one cavern layer to the next. The top z-level, where the pit meets the next cavern level, is un-muddied rough rock floor where the normal space of the deep pit and the random rock spires of the cavern collide.
Deep pits act as a flyer-only passage for lower-level creatures to path to your fortress, which is a bad thing. On the other hand, they can be used for garbage disposal and in retracting-bridge traps, as it's a long, long way down to the bottom, and will render anything that falls through into bloody splatters.