Nazi pseudogeography borrowed heavily from Norse mythology and Tibetan folk beliefs. The pagan Norse believed in a world below ours where really evil dark elves lived while the Tibetans thought that the Earth had openings to worlds below where one's soul could achieve nothingness.
The Nazis, thinking themselves enlightened, rejected the supernatural elements of these beliefs while clinging to the hope that there were worlds inside the Earth. Hopeful the worlds would have easily reachable resources and having Aryans or easily conquerable people living below certainly would have been a plus.
While the Nazis did claim a section of Antarctica, New Swabia, there was no expedition there to find a hollow Earth. There is some evidence that the Nazi Tibetan expeditions were looking for portals but only the most die-hard occult Nazis thought it was possible to locate one.
The plan to conquer the hollow Earth never left the fleeting minds of the mad men who thought it up.
Thule Equals Aryans Around the World
Holding slightly more weight in Nazi pseudogeography. The Nazis were genuinely interested in researching their supposed Aryan heritage with organizations like the Ahnenerbe. Portions of the society though were infected with the thoughts of the occult Thule Society. The society believed Aryans did not some tan browned-skeined, brown haired people but were in fact blonde haired, blue-eyed whites from Atlantis! After the Aryans managed to lose Atlantis they fled to the lost land of Thule, the society believed. Once on Thule the Aryans supposedly left and easily settled across the world while the rest of humanity was living in the Stone Age.
The Nazis in general believed that an Aryan people once roamed over a wide section of the Earth. The Ahnenerbe supported this belief by pointing out that Far East people like the Kalash and Ainu had Nazi-prized Caucasian features. Research trips to places like Tibet sought to further back-up the Nazi's point of view. The Nazis failed to anticipate the discoveries in genetic geography that mapped human expansion out of Africa which links all humans together. Also, Caucasians flowed both ways, into Europe and into Asia. It was in Europe where the race fared better against Turkic and other groups.
The Nazis had much more down to Earth views that did far more damage. However, a pseudogeography in particular and a whole range of pseudoscientific thoughts were created and used by the Nazis to justify policies that lead to tanks rolling and people dying.