From publisher blurb:
This expansion describes the undergrounds of the city of Wastburg. Characters will discover the quarries that gave birth to the city, Werdenburg the city-under-the-city and all the factions, losers, smugglers and people of ill repute that uses those tunnels. Also, more information about the cities of mud, the slums that survive in the cracks of city.
Also included is a scenario that sends the guards in the underground and a few synopsis of stories to continue using the tunnels.