From the back cover:
Deep below the streets of New York City, there exists a world of wonder and adventure, unseen by those above. Magic courses like blood through veins made from steel tracks and transformers. The UnderWorld is home to the Homeless, to the Freaks, to the Junkmen, and more. Steely-eyed Bravos hunt their prey in dark tunnels far below the subways, while brave Taggers scout new paths between the varied Domains, and mark the trails for all to follow. Creatures of Legend walk the platforms below the city, and brilliant Artificers build inventions of bizarre science, borne of madness.
This is the world of heroes and villains... of magic and romance... of gods and monsters.
Unseen. Unknown. Unforgettable.
UnderWorld is an adventure game, usable for either table-top or live-action play.