I’ve really been digging into the pulp roots of comics for a while now, and a major overlap between “pure pulp” and superhero earths is the existence of a Hollow Earth, or something like it. I’m curious about this on Sentinels Earth. Insula Primalis certainly hints that something like that could exist. Are there mole men and lava monsters and stranger things that hail from beneath the ground? Have people been stumbling into the Hollow Earth for ages and intermingling with the indigenous cultures? I’d love to see some relic characters and places from the Golden Age show up. It’s certainly a trope in comics; why not here?
This brings to mind the Myst books to mind as well as Chronicles of Narnia: The Silver Chair, many possibilities with that idea. “Many go down and few return to the sun lite lands of the world above.”
I would say it’s likely Akash’Bhuta exists in a similar realm during her “off days”, meaning when she’s not actively attacking the world.
Thanks, Adam. That’s actually what got me thinking about it. 
Night Mists back story strongly suggests that this game takes place in a universe where Arkham Horror also took place, so it would seem that many of HP Lovecraft’s hidden worlds would be fair game too.