When life in the great ruined city became overbearingly hostile, some survivors crawled into the the darkness below. It turns out that things are tough all over, and life isn't greener on the other side of the asphalt.
The underwellers are driven to the surface, and to madness, by extreme hunger, Resources below the surface are nearly depleated, resulting in savage fighting between enclaves. While the surface poses many threats to the underdwellers, it holds the promise of meat. You ar ethat meat.