Cryptic Ruins of the Deserted

A network of tunnels hiding a secret history of New York and a South African ghost town are now mysteriously abandoned ruins; experts investigate the cryptic secrets of these deserted places to uncover how they were built and why they were abandoned. This is a mashup of S05E04 "Kingdom of the Vanished," S03E08 "Nazi Death Star," and S03E01 "Doomsday at Dragon's Teeth."