An underground civilization may exist, even without "Hollow Earth" theory. They may live in the Earth's crust.

Okay, the "Hollow Earth" theory is mostly bullshit.

Earth is not "hollow".

But that DOESN'T mean that underground civilizations can't exist!

An underground civilization may exist, even without "Hollow Earth" theory. They may live in the Earth's crust!

An advanced civilization may live in a complex system of caves and tunnels inside the first 5 thousand meters of the Earth's crust.

Thay may get their energy from a geothermal extraction process, and get their water from the subterranean aquifers.

Thay may grow their food in advanced greenhouses with ultraviolet bulbs...

Maybe their population is reduced, no more than 100,000 to 200,000 individuals.

And maybe this underground civilization are the so called "demons" from the "underworld".

All of this is possible, even if the Earth is not "hollow".