Other speculation regarding Byrd’s supposed search for Agartha notes a Nazi presence in arctic territory, both North and South. It is well-documented that the Nazis explored arctic regions to set up bases and test novel weaponry, but it is also well-documented that Hitler and the Nazis were obsessed with esotericism and the occult.
The rabbit hole goes deep here, with some going so far as to posit that Hitler could have escaped to this underground world, though that might be counter-intuitive if an ancient enlightened society resided down there.
Nazi maps believed to be instructions for reaching Agartha have supposedly been corroborated by a letter from a German U-boat navigator named Karl Unger, who claims U-209 made it to Agartha and that the Earth is in fact, hollow. The letter also mentions the notorious German Gens. Karl Haushofer and Rudolf Hess, who he says were correct about the hollow-Earth theory.
There is ample evidence the Nazis spent a lot of resources searching for Agartha as a last resort for Hitler to escape in case of dire emergency, somewhat warranting these conspiracies. In fact, the most common diagram of Agartha was drawn by a German scientist in 1935.
Evidence also exists to indicate the Nazis may have believed we already lived on the inside of the Earth. Some experiments of the Third Reich show a belief the Earth was concave, and that we reside on the interior of a sphere.
In a bizarre 1942 experiment conducted by Nazi scientist Heinz Fisher on the island of Rügen, a team attempted to observe a British fleet by pointing the lenses of their infrared equipment up at a 45-degree angle. They believed that, though the Earth was concave, normal rays were refracted off the ocean, but infrared rays were not. Of course, they saw only the sky, but this shows that they believed and searched for evidence of alternative Earth theories.