Machu Picchu was surrounded by roughed, inaccessible mountains. The ruin was kind of intact. Bingham said it was never found by the Spanish and it was a secret holy city or a "lost city". But it's actually more of a "abandoned city" than a "lost city". Some archaeologists said that the Spanish did record the existence of Machu Picchu. The Incas set fire on the city when the Spanish came. And the fact that most "holy city" in the world are usually not very "secret" makes Bingham's report seem doubtful (if it's secret, how can you market your religion?).

But still, something remain very secretive in Machu Picchu, e.g. 70% of the human remains found in Machu Picchu were females. And there are lots of interesting gimmicks around the city.

Reasoning of the idea: The Incas believed that life originated from Pacha Mama, the mother earth. That's quite true, according the Christian's belief, men and women were made of earth. Even scientifically, life was formed because of some in-organic materials (e.g. soil) interacting with lightening. It made sense to them that if we take from mother earth, we would need to return something back to mother earth. That's why some human sacrificed or mummies (naturally dead) was wrapped in a fetus position (you go back to the same way you came from).

It was not known that Incas have knowledge of the compass, how they figured that out was a mystery.

Another interesting thing about the sundial was at noon of June 21st, the upper rock will cast no shadow upon the lower rock. This usually only happens to places on the equator (because the sun was directly above). Obviously the rock was tilted in a very precise position to achieve this effect.