The trash had slowly piled up. It took over, piece by piece. And no one seemed to notice until it was too late.
Those with money used it to simply move their houses to the top of the heap, compacting it. Turning it into stinky, solid foundations. And yet, this did not deter them from dumping garbage out their windows.
For those with meager means, they were forced underground. Into the sewers.
The fecal matter had long since dried up as the garbage slurped up every inch of wetness that came it’s way.
Food and water were scarce, causing many to die of starvation.
Clothes deteriorated with nothing to replace them. Some died of exposure.
Space was limited which led to feuds. Many perished. Eventually, there were only a handful left. The hardiest of the human race.
They lived in peace after that for decades. Procreating only occasionally. But those progeny were unlike any that had come before. Lean, fit, and able to withstand much. And they were about to unite and declare war on all surface dwellers.