Off the island of Pohnpei in Micronesia, lies the ancient city of Nan Madol, the only ancient city ever built upon a coral reef. Comprising a set of almost 100 stone and coral fill platforms atop artificial islands separated by narrow channels and enclosed by an outer seawall, Nan Madol is an engineering marvel. Yet despite the enormity of the undertaking in building the city, no records exist as to when exactly it was built, where the enormous rocks came from, how they were transported there, and for what reason it was constructed on top of a reef. Natives of the island claim the extraterrestrials or people from the sky came and built this ancient city. Ancient Astronaut theorists Giorgio A. Tsoukalos and David Childress explore the ancient city to find some answers to this mystery.