What's Up with the Bottomless Pit?

d12
1. Hell's exhaust vent, spews forth occasional gouts of variously offensive, potentially lethal smoke, vapors, gases, really does lead (eventually) to Hell
2. Course of trans-dimensional amusement park-style ride, while falling forever 1% chance of encountering pod full of delighted screaming children of eldritch beings zooming by at astonishing speed
3. Dry bed of extinct interplanetary energy river, former trade route from time before the Sundering of Worlds
4. Soon it will serve its original purpose of excreting incomprehensibly alien life forms into the world, but don't worry, it's all part of the gods' plan
5. Inter-planar "doggie door" left open to allow monstrous alien pets to stretch their ambulatory organs, romp around campaign world
6. Underworld agency uses living things caught in series of sieves as raw materials for deity under construction
7. Massive outer entity died, collapsed into singularity, sank to world's core, hyper-intense gravity increases continuously the further one goes, effects of which endlessly fascinate evil sorcerers everywhere, multiple research projects underway at any given time
8. Left over after reckless detonation of experimental homemade wormhole bomb
9. Actual source of atmosphere on campaign world, stolen from gas giant by works of forgotten Agent of Creation
10. One of innumerable fuel intakes powering spaceship engine in adjacent dimension
11. Gateway to alternate Utopian campaign world of excruciating tedium and an almost complete lack of adventure
12. Esophagus of the Famished God, brain-damaged cultists charged with making sure tasty things get tossed down there 24/7