In Alaska, the ruins of a massive compound point to eerie evidence of an apocalyptic mystery. New discoveries reveal why this strange marvel was built in the snowy wilds of the Last Frontier and why it was abandoned.

Near the destroyed city of Pompeii, an eerie complex of underground tunnels snakes beneath a volcano, and some believe these passages were intended as a gateway to hell. New discoveries reveal exactly why they were built and why they were abandoned.