Ant Nursery (3.5e Equipment)
From D&D Wiki
CR 1; mechanical; location trigger; no reset; ten feet deep (1d6 fall); Reflex DC 15 avoids; Search DC 20; Disable Device DC 20. Market Price: - gp.
This isn't actually made as such or quite a trap in the normal meaning, but is instead a natural occurrence that people who know about it might be able to use as one. The nursery ant is known to build the largest underground excavations of any insect, their nurseries can be up to 11 ft wide under ground chambers that they hollow out often just a few feet bellow the surface. If enough weight passes over these they will collapse, leaving the unlucky victim in a pit filled with furious biting ants (1 point of damage per round). In reality this has been the fate of many unlucky farm horses, which upon falling in were unable to escape the pit. Any character falling in will have to climb the crumbly dirt walls while enduring hundreds of stinging bites (Climb DC 10).