Yongaguni is at the very Southwestern end of the Japanese archipelago, close to Taiwan. There is an underwater archaeological site with paved roads and stone structures that make you think of Machu Picchu.
This is an area that has been underwater since the last ice age, 12,800 years ago. The stones can't be dated (only organic material can be carbon-dated); but we know that the the construction is more than 12,800 years old by the fact that it is underwater. This is a big deal, because in the "standard history of humanity" there was no technology to build large stone structures before the Egyptians, who are said to have built the pyramids 5,000 years ago. Some rogue archaeologists think they may be much older.
Hancock's hytpothesis is that there was an advanced civilization with global reach in the deep past, that it was wiped out when the last ice age came to an end with a humongous flood (memorialized by Noah and other traditions around the world), that not much of the technology survived, but that hunter-gatherers in many sites around the world were better equipped to survive the disruption, and it is from them that civilization arose anew.
Machu Picchu means "Old Mountain", and Huaynu Piccu is the "New Mountain" behind it. The New Mountain has Inca construction from before Pizarro wiped out their civilization in 1534. The Old Mountain contains stone construction in a similar style built atop foundations that were far more sophisticated. Rocks in the foundation layer were finely carved and fitted tightly together, though no two are exactly alike. They were up to 20 tons in mass, comparable to the largest stones in the Great Pyramid in Egypt.
In this wall fragment, it is easy to distinguish the architecture of the old megaliths at the bottom, and the cruder Inca archaeology built on top of it.
Brien Foerster's guided tour makes a compelling case that there are two architectural technologies represented here, and that the advanced technology is far older, a relic of a (possibly global) civilization that was destroyed by a cataclysm at the end of the last ice age. (This event is described by Graham Hancock as a meteor impact that instantly melted the vast North American glacier, and also sent clouds of dust into the stratosphere that cooled the earth for hundreds of years thereafter).
Our economy and culture today are globally interdependent. It is likely that if chains of communication and transport are destroyed by a natural or man-made catastrophe, the world's vast, urban populations would not survive, but primitive hunter-gatherers who live close to the land and rely only on local resources might survive to seed civilization anew, perhaps with a few stragglers from the world of modern technology keeping alive bits of our sophisticated but fragile culture. Hancock speculates that this is what happend 12,800 years ago.