Bottomless pits are, in fact, bottomless. Adventurers that have fallen into these traps have been known to meet other adventurers that have also fallen within, and even marry and bear children before eventually dying of old age.

Prior to Curse Day 883 GUE, bottomless pits were the second-leading cause of death in Flatheadia. The Frobozz Magic Bottomless Pit Bomb Company produced anti-pit bombs which were able to stack a bottomless pit full of bottomless-pit-filling agents. On this Curse Day, the man who would become the First Dungeon Master tossed one of these into a cavern containing an incredible quantity of these pits, which happened to be the resting place for a majority of the surviving grue population. Uncountable hordes of these creatures emerged. Thus bottomless pits were no longer a leading cause of death.

In the intervening years between 883 and 948 GUE, at some point the implementer, Bruce Daniels, worked for weeks to fill in almost all the bottomless pits in Zork, thus forcing even more packs of grues to slaver about.

SOURCE(S): Dungeon, Zork Zero, Zork: Grand Inquisitor