This episode is 2-hour a mashup of S04E07, "Alien Hell Portal", and S04E04, "Labyrinth of the Dead".

A strange complex of buildings on a remote tropical island was once home to a large seaside community, but today, locals avoid the area and claim that it's haunted. New discoveries reveal why this eerie place was built and why it was abandoned.

New discoveries reveal why the world's most ambitious engineering projects were abandoned, unraveling the dark mysteries and bloody secrets of these strange ruins.

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.

A strange complex of tunnels on the grounds of a bizarre mansion were under lock-and-key for a century. Since its rediscovery, some believe its mysterious place is a gate to hell. New discoveries reveal its secret purpose and why it was abandoned.

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.

A ghost town in the American West was once a Gold Rush hotspot and one of the most violent places in America. Today, there are legends of a secret treasure buried in the surrounding hills, and new discoveries might finally reveal its true location.

Tunnels filled with human bones lie beneath the streets of Paris, and this horrifying engineering marvel points to a dark secret; new discoveries reveal why this grisly underground death maze was built and why it was abandoned.

The ruins of a once-palatial compound contain the secrets of one of history's most notorious criminals, and new discoveries reveal why this high-security estate was built and why it was abandoned.

On a strange island off the coast of Croatia, the mysterious ruins of a long-abandoned hideout look like they once belonged to a James Bond supervillain, and new discoveries reveal why it was built and why they were abandoned.

The world's strangest ghost towns are home to dark secrets, creepy buildings, and eerie sights, and new discoveries reveal why they were abandoned.

Some of the world's most incredible engineering projects now lie in ruins; cities that are now empty ghost towns, bridges that were never finished and roads to nowhere; these deserted places are home to extraordinary mysteries and untold secrets.