Agartha
Agartha is a purported mythical realm located underneath Kern on the inside of the hollow earth. To enter it, supposed passageways on the earth's surface leading deep through the earth's crust eventually reach down onto the floor of Agartha, at which point gravity reverses, and as such the entire realm is upside-down from the surface's perspective.
No adventurer group or expedition led by academia has ever located a real entrance to Agartha in modern times, but its existence is attributed to the works of dozens of historians, poets, and cartographers from diverse geographic locations during antiquity.
Features
Agartha is almost always described as being a hyper-advanced, utopian civilization inhabited by humanlike creatures underneath a bronze sun. Due to the realm's location, it is permanently daytime, so their inhabitants have adapted to never need to sleep. Some historians called these creatures "nephilim",