After World War II there seems to be a continuing connection between hollow-earth stories and Nazi Germany. Zundel also suggested that the Nazis had originated as a separate race that had come from the inner-earth. As time has gone on the idea of a hollow-earth has become less a theory of fringe science and more a subject of science fiction and fantasy. Perhaps this has happened because new discoveries continue to show there is no validity to most of the hollow-earth ideas. United States Navy Admiral Richard Byrd flew across the North Pole in 1926 and the South Pole in 1929 without seeing any holes leading to inner-earth.
Photographs taken by astronauts in space show no entrances either.
Modern geology indicates the Earth is mostly a solid mass. One believer did seize on a NASA photograph showing a black hole at the North Pole and called it proof of an entrance to a hollow-earth. As it turned out the photo was actually a composite of several pictures taken over 24 hours so that all sections were seen in daylight and the black hole at the top was the portion of the arctic circle never illuminated during the day over winter months. The book illustrates a third theory of hollow-earth which is more plausible than the other two. This is that passages from the surface lead to caverns underground in which life thrives.