Why is there an Underworld?

d12
1. Earth hollowed by extinct giant ants during paleogean Age of Giant Insects
2. Life began in crucible of chaos at center of the earth, only breaching the surface after millions of years in the dark oceans below
3. Groundwork laid by primeval visitation of planet-eating cosmic worms
4. Aboriginal population of mole-men warped into surface races by renegade god
5. Surface evacuated by ancient ones after their twisted sorcery caused the atmosphere to catch fire for 1000 years
6. Planet strip-mined by aliens, totally depleted of element vital to interstellar travel, abandoned
7. Because of a lie the elf fathers told the first dwarf
8. The evil gods imprisoned at the earth's core call out to be freed, their children delve ever downward
9. Fossilized circulatory system of the dead earth god, upon whose corpse we all feast
10. Down below where the goblins and kobolds dwell the god of retribution fuels his spirit-powered engines of anguish with wicked souls
11. Provision of First God War armistice created a permanent division between children of light and darkness, since nullified by renewed hostilities
12. Underground realms left over from first major revision of creation when mountains rained from the sky to forever bury failed experiments