I mean, one of the big problems with a sub-terranian environments and ecosystems is that it is cold down there. Like, we constantly underestimate how cold our world would be without the Sun constantly trying to set us on fire 24/7.
Go stand outside in your under-roos at midnight at the height of winter. Then magnify that by a factor of 20, that’s how cold 10 feet under the earth, with no immediate openings to the surface, gets.
Now think about how far underground we are.
If the Beledar Crystal ever goes out or goes completely dormant, that entire sub-terranian ecosystem, both Hallowfall and Spuder-Town, will go poof. The only reason we’re not walking into cold, sterile, damp caverns is because of plot logic.
By contrast, the caverns beneath the Dragon Isles would have been much more inhabitable due to volcanic activity providing heat, light and chemical reactions for basic life-forms to survive, and assuming there were enough passages to provide air-flow, its possible more advanced kinds of life could have either developed, or migrated, to that underground system. And considering that Fyrakk only bored his way in from one direction, the other entrances must have been naturally formed, or a hold-out from Neltharion and his brood.
A ‘realistically’ useful underground biome would require multiple large openings to the surface, plenty of water, a much hotter surface temperature, small but persistent volcanic activity to function, and the likely-hood of sapient races developing naturally would be even lower than the odds of our own evolution.
I would crack up, however, if one of the places we have to go to deal with Xala’toesies is one of those floating glaciers up to the north, beyond Northrend, similar to how the Death Knights got their Legion Class Mount … and considering how many glaciers have a much more significant amount of their mass underwater, finding a Titanic facility buried in the ice, on purpose, and left to float to avoid it being easily tracked while operating in a low-power mode to reduce detection, would be amazing.
The Titanic structure in the heart of the iceberg could be our main base, with caverns either naturally forming in the ice or carved out through specially modified Goblin/Dark Iron mining machines to get to disconnected power-batteries or similar situations could be a fun little mico-area to visit, much like the up-coming Siren Isle is going to be. The intense cold, the darkness of the deep ocean around you, the lingering Scourge who do not need to breathe trying to claw their way in, and the Void-in-the-Shadow that haunts the Kobolds has decided to follow us instead for the amount of death and blood we leave in our wake would make a fascinating sub-plot, something similar to the Thing but supernatural rather than science fiction.
