Some musings on the etymology of Agartha

In the US Nephilim rulebook and supplements, "Agartha" is the perfected spiritual state that Nephilim seek to achieve. Although there are stories of Agartha being a physical place, a city located at the center of the Hollow Earth, these are lies concocted by malevolent secret societies to seduce initiates. 

In the French version, the mythical city was later introduced as "Aggartha" (with two Gs). This was the headquarters of the World Arcanum, and thus a play on the wording of "center of the world". It was not located in the center of a Hollow Earth, but was located in the astral plane.

Anyway, the real life occultist description of Agartha mentions it is a city at the center of the Hollow Earth and lit by an inner sun (in some accounts a "diamond sun"). What does this inner sun remind you of? It reminds me of the "Grail", the game's name for the Earth's core where the planetary Ethers are transmuted into the magic fields. (In the French version it is called the "primordial grail" and "celestial athanor".)

So the idea I had was to equate the Hollow Earth with the Subtle Planes of Agartha. To some degree this is already done in the French version, as Aggartha and the lost continent of Mu are subtle planes, but I could only find a handful of references to the Hollow Earth concept.

So the Grail, the inner sun, is where the planetary and solar ethers are transmuted into the magic fields that bathe the Earth. From there, the Subtle Planes exist inside these magic fields, orthogonal to the physical world. This is where the mistaken idea of a Hollow Earth originates: the subtle planes of Agartha are not physical places inside the Earth, but spiritual realms within the magic fields that interpenetrate the interior and exterior of the Earth. 

Although Agarthans are the most well-equipped to travel the subtle planes, other Immortals and even mortals are capable of doing so. They can do so by astral projection, the safest method, or by finding and entering the various portals to the subtle planes. Entering a portal is very dangerous: without a physical body to anchor you on Earth, upon death on a subtle plane your soul will become a ghost trapped in the astral... or worse. Physically entering portals to visit the memory of past ages is probably where the pulp fiction idea of a Hollow Earth containing bubbles of prehistory originates.

In a funny coincidence, this concept I just proposed closely resembles the design of the Hollow Earth in the Monsterverse. There it is depicted as existing parallel to Earth via higher dimensional geometry, rather than violating our modern scientific understanding of the Earth's interior.