Old City - the oldest part of Newford, dropped underground at the turn of the century during the Great Quake that destroyed much of the town. The survivors built on the ruins and so Old City remains hidden underneath the new Newford. There are a number of entrances, most of which have been closed off by the town authorities, to avoid the "undesired" part of the population hiding away. This has of course failed, and a number of entrances are still known to the natives.
Rushkin hides here with his Numena at the end of Memory And Dream.