Part One:
Aside from the 'Sky God(s)', UFO-nauts, and all that stuff, the actual and very real underground city carved out of the soft stone of Cappadocia is quite interesting. Whether these chambers were built as a refuge against invaders or to hide-out from disastrous environmental events, to escape from warring extraterrestrials, or something even more plausible, they really are down there and this is something that could be brought into an RPG setting from deep antiquity to far future, and it could easily be adapted to alien worlds as well.
The connection to the American southwest caught our attention. Do these underground cities have some so far unexplored/unexplained connection to that gigantic Krell-inspired secret base overseen by the Tic Toc Corporation?
The Native American myths/tales/accounts of living underground, warring aliens, ant-people, snake-people, etc. are also quite intriguing...
You don't need the UFOs to make this work, but you get them lumped-in anyhow as a sort of bonus.
Part Two:
Bingo! Now we get Conspiracies, aliens living underground and doing vivisections of unwary visitors, government cover-ups, and more. Pure RPG gold! But the Southwest isn't enough--we also get the Andes to go investigate at Erich Von Daniken's behest. Hall's actual-factual 1976 expedition had 100 guys including an American astronaut, and they went down into those caves looking for golden artifacts of possibly alien origin...they didn't find the golden treasure, or did they? The Metal Library is too good an idea not to use it. And then there are all those dead explorers...what if some disreputable necromancer was digging them up to interrogate their corpses about the truth of these treasures, 'failed' expeditions, etc.? Creepy, weird and just pulp-enough to be a lot of fun...
But wait, there's more! Elongated skulls. There 'might' have been some sort of connection between the old cultures of South America and ancient Egypt, with the space aliens forming a trifecta of cool, pre-historic weirdness that could easily lead into Churchward's Lemuria, Donnelly's Atlantis, Cayce's Bimini, Blavatsky's Root Races, Velikovsky's whole Worlds in Collision thing, Sitchin's Annunaki and more. Plus you get humans or proto-humans or hybrids with elongated heads...
What Fun!
Part Three:
Cenotes in the Yucatan. A cave like no other. Mysterious underground temples out in the jungle, some submerged underwater. Broken columns with strange inscriptions deep under water leading to massive underground temple complexes...now that's cool!
Stalactites sharp enough to slice through flesh. Cold, dark, hordes of bats, the whole trip through the chambers to Xibalba sounds like a megadungeon entrance...
A 'Road to the Stars' could be interesting. It may or may not be clear that anyone has arrived from outer space, but it is a rather intriguing reference that could easily inspire some extraterrestrial shenanigans for those elongated skull people, and other, possibly competing human-related and non-human parallel civilizations...
And then we get to the North Pole and the Hollow Earth. Yay! (about time...)
Edmund Halley took this theory seriously. It explained how the magnetic poles could shift. Then he went farther, speculating about multiple nested spheres and who might be living down there.
John Symmes gets a mention, but they gloss over the good Captain so we'll return to him in another post.
Verne's novel 'Journey to the Center of the Earth,' gets a nice nod. But Verne's fictively-useful notion of extinct life forms surviving deep down below in the hollow earth is cool (Burroughs made it a central part of his Pellucidar series), but eventually dry geology lectures and the allure of dinosaurs down below loses out to advanced ray-gun wielding psychically-empowered technological civilizations with flying saucers that got all the publicity and coverage in pseudoscience and fiction. The producers of this video really gloss over a lot of stuff at this point, missing out on Shaver's derro, glossing over the Mt. Shasta channelers and other folk who claim to be in communication with telepathic beings living deep within the hollow earth, missing out on the non-epic that is Gene Autry's The Phantom Empire...and they miss out on the Vril and all those connections as well...and instead leap ahead to Admiral Bird and Operation High Jump...an actual, real-world expedition that bears a lot of similarities to At The Mountains of Madness...
All in all, a nice bit of brain candy for developing loads of RPG stuff. Just keep your salt shaker and tin foil hat well within reach...