Nuclear armageddon, biogenetic holocaust, meteorological catastrophe, magnetic shift, deep impact, environmental devolution. The nature of the Cataclysm is lost to history. Something happened, and the world, as they knew it, was gone.
Ironically, the powermongers who had (according to some scenarios) caused the Cataclysm comprised the majority of the survivors. Stripped of their mindless consumer flock, these evil shepherds retreated deeper into their vaults as the fires of the Cataclysm cleansed the Earth above.
Through the turmoil of the Cataclysm, those without shelter, those few who survived the initial apocalypse, carved their futures from the bones of civilization. Lone survivors confronted, conflicted, united, raised families, settled into communities. Slowly, the Earth was healing.
As the pioneer revenants of the Wasteland rebuilt, the "worms" crawled upward. They did not stray far from their vaults, building cities around their entrances. They had retained networks of communication with vaults around the world, and began to rebuild commerce between them. Cities rose from the ashes.
Initially, the cities were heavily guarded against "visits" from without. Their leaders were certain anyone surviving the Cataclysm "naked" must surely be some kind of monster. All prejudices are grounded in some fear of the truth, of course. There were "monsters" in this new world. Many of the outsiders bore progeny "monstrous" by old world standards. Something had been mixed with the stuff of humans -- something of gods and demons.
The cities were not without their own monstrosities. While those outside were borne of nature, the strangeness within the vaults was borne of human lust and greed. Entire races were bred for the purpose of serving their masters. But for their chains, these races were not entirely unlike some of the new peoples in the Wastelands.
Eventually, the two worlds mingled. Trade is vigorous, but the rulers of the cities are careful not to let too much of their technology into the hands of the "savages". Remarkably, they have made very few attempts to colonize beyond their walls. On the contrary, they have taken steps to control their own population's growth, perhaps heeding at least one lesson taught by the Cataclysm.
Contrary to the sterile society of the cities, the outlands are teeming with life. By definition, the Wasteland is "sparse" (to say the least) but there exist many oases of wildly varied civilization, many unaware of anything beyond their own boundaries.
Technology, myth, magic. It is a new world of wonder, and it is yours to explore.