World Map Information

List of Underground MapGen
( 6 ) Geomoria - ? -
( 7 ) Underworld -17000 till -150
( 8 ) Caverealms -28000 till -17000
( 9 ) Fun Caves + Underworld -31000 till -28000
What exactly is meant by underworld? Caves? Netherworld? Something else?
Also, what is Geomoria? A fossil cave or something?

The solution is simple: Go and explore.