The first rural cemetery west of the Mississippi is home to the elaborate tombs of St. Louis' local beer barons.

The "Greatest Sewer" of ancient Rome is one of the oldest sewer systems in the world, and is still in use.

St. Francesca Romana, patron saint of drivers, resides in the church she founded.

This ancient Roman prison is decorated with an upside-down cross not as blasphemy but as saintly tribute.

Jesus' footprints and the first catacombs in the world.

One of seven Jewish catacombs in Rome and one of only two open to the public.

At the crypt of St. Mary of Eulogies and the Dead you are left alone to ponder mortality among piles of skulls.

Hidden beneath the famous St. Peter's Basilica is a veritable city of the dead that may hold the remains of St. Peter himself.

The underground sanctuary of a mysterious ancient cult.

The engineering wonder and pleasure dome of a much-maligned emperor still holds undiscovered secrets.

The crypts of Capuchin friars decorated with the bones of over 4,000 friars, including an entire "crypt of pelvises."

A island reachable by a causeway at low tide is possibly one of the earliest Western European locations to be identified in text.

This tiny island featuring 9th-century Viking ruins is only accessible at low tide.

A rocky sea serpent hides caves, blowholes, and a spectacular natural bridge.

Accessible only at low tide, the deserted island is dotted with World War II ruins.

Take a sea tractor to the Art Deco hotel on the island that inspired Agatha Christie.

A rare opportunity to drive across the ocean floor.

The Atlantis of Cape Cod reemerges during the lowest of tides.

This 1,300 year old monastery built atop a single rock was once only accessible depending on the whims of the tide.

This submersible road connecting a French island to the mainland is lined with safety towers for when the tide strands travelers.

Sometimes an island, sometimes a rocky outcropping connected to the world by a tiny suspension bridge.

Twice a day this ancient path disappears with the tides, leaving the Holy Island of Lindisfarne out to sea.

The abandoned personal amusement park of the King of Pop.

A crumbling wizard is one of the final reminders of a roadside Middle Earth.