Yeah, sometimes. Sometimes it's other supernatural beings like angels or beings of light. http://www.crystalinks.com/hollowearth.htmlI've always considered the idea of a 'Flat Earth' to be just an adventure into either bad history or some kind of favorite pet project of a pseudo-scientist with too much time on his/her hands. Afaik no one has believed in a 'Flat Earth' other than the slow class since the Greeks figured out the planet was a sphere way back in the day.
And Nazis beneath the surface of the Earth? Sounds like a variation on the same BS that says that Nazis live on the Moon... Does this 'Nazis down below' BS involve UFOs?
Forum for people who believe in flat earth theory: https://www.theflatearthsociety.org/forum/