In the future, the underworld with the Calamity mod and Elements Awoken mod installed will be an overworld of its own.
Elements Awoken Hell or High Water update will introduce 3 new minibiomes to the underworld and Calamity devs are planning to split the Brimstone Crags into three minibiomes. That makes 7 distinguishable 'zones' for the underworld. I am all for this.