The floor of this spectacular Baroque sanctuary is covered in memento mori.

The oldest surviving synagogue in New York City is now a soaring art space.

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.

The underground sanctuary of a mysterious ancient cult.

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

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

This beautiful building in the heart of Manhattan's Financial District was empty for decades.

This sculpture by artist Fritz Keonig survived the 9/11 attacks and now stands as a monument to the victims.

This little museum is built on a theater that was once a wildly elaborate speakeasy.

Collapsing castle on the Hudson river was once the storage site of military surplus business.

This abandoned subway station beneath the Waldorf-Astoria hotel is allegedly used to secretly transport presidents.

Tucked in the back of this amazing antique store is an important piece of medical history.

Amid the bustle and noise of the Financial District hides a secluded garden oasis above the city streets.

Dodge prohibitionists and slide through the telephone booth in Crif Dogs.

A beautiful and abandoned New York subway station from 1904, complete with chandelier.

New York City's only preserved family home from the 19th century.

During the holidays, a vintage subway train whisks New Yorkers back in time.

Though its celestial ceiling is impressive, the true star is the vast collection of French books.

This Wall Street bar was once a hub of revolutionary activity where Washington famously bid farewell to his troops.

This cemetery has graves dating back to the 17th century, including the city's oldest carved tombstone and an ominous cryptogram.

A crumbling hospital from the 1850s on Roosevelt Island.

A terrific purveyor of natural history objects and curios.

Manhattan’s Battery Park has a whimsical carousel full of spinning, glowing fish.