“There are cavern systems and caves that traverse the whole mantle,”
https://in5d.com/hollow-earth-the-hole-truth/
"there are deep systems of tunnels we don't know about - the Honeycomb Earth)"
https://m.imdb.com/list/ls003260126/
"Other smaller entrances scattered about the Earth's surface leading into great tunnel systems connecting with the cavern world inside the Earth"
https://archive.org/details/SecretOfTheAgesUFOsInsideTheEarth
"The earth is connected by a series of subterraneous cave systems which center at the North Pole where a small circular gap can be found leading to inner earth."
https://www.reddit.com/r/HOLLOWEARTH/comments/p6r86f/the_earth_is_hollow/
"The idea is based on the ancient legends of many cultures that say there are races of people – entire civilizations – that thrive in subterranean cities."
"Very often, these dwellers of the world beneath are more technologically advanced than we on the surface."
https://anomalien.com/secrets-and-legends-of-the-hollow-earth/
"According to the book he travels to Hollow Earth through a cave that is part of Mammoth Caves in Kentucky, USA and along the way he learns many ancient secrets and the reasons they have been withheld from mankind for so long."
https://discover.hubpages.com/education/Hollow-Earth-Reveals-The-Truth
"The Hollow Earth Theory dates back to ancient times, where it was part of mythology and legends. Given the Earth’s long and deep unexplored cave systems, it shouldn’t come as a surprise that people have projected so many ideas about what lies beneath the surface.
Ancient Greeks, among others, claimed that there were small caverns in their lands which were in fact entrances to the underworld. Numerous myths and legends mention portals to inner Earth and heroes living under its surface."
https://hypedepot.co/hollow-earth-theory-explained-just-a-conspiracy/
Macuxi Indians claim Earth is Hollow and inhabited by “Giant Creatures”
https://www.monkeyandelf.com/macuxi-indians-claim-earth-is-hollow-and-inhabited-by-giant-creatures/