I put this in my intro for Jelly Jam, but I hope I'm not taking too many liberties with this just the same.

Underground Slime City:

About the size of a large city, it is mostly composed of metallic buildings and streets that were uncovered from layers of rock and soil.  Much of the city is still unearthed.  It is located a few miles below Void's subway system and even goes under some of the more watery regions on the surface.  With numerous supports in place, the risk of a sinkhole forming is minimal.  The general population seems to be around 65% slime monsters, 25% mole people, and 10% other.  The city is considered part of the surface country, but has its own mayor and town government apart from Void City.  So far, the only known passage way between Void and Slime City is a strange looking overpowered elevator that goes from downtown Slime City to an alleyway in Void's port district.  Because of the limited access between the two cities, interaction between the populations is limited at best.