I didn't actually grow up in the Undercity - it should be noted. Just on Coruscant. So the Underworld is actually under the surface? Cause the Undercity is technically on top of the surface - it's just under a lot of other stuff.
The galaxy is a big place and there's a lot in the Unknown Regions that nobody knows anything about so I've very likely just not heard of it... What do you mean predate it?