Caligula's colossal pleasure ships, discovered by Mussolini only to be lost in flames.

A medieval stronghold rising over the ruins of ancient Roman villas along Rome's southern coast.

A fascinating assortment of cave dwellings, medieval ruins, and sarcophagi are being devoured by the forest.

This ancient altar emerges from the thick forests to reveal the mysteries of Etruscan sacrificial rituals.

Italian director Pier Paolo Pasolini fell in love with this lonely 13th-century tower while making his most famous film.

Largest necropolis in the Mediterranean.

The oldest house in Manhattan is now a museum that remembers the home's sordid history of scandals, vice-presidents, and ghosts.

A no-frills cinephile haven known for showing offbeat, underseen films.

Empty, but not abandoned, this cavernous Brooklyn loading dock was once considered the largest individual building in the world.

A discreet little stairway on St. Nicholas Avenue leads to a quaint and unexpected cobblestone street of 19th century wooden rowhouses.

Nikola Tesla's former home contains an homage hidden from plain sight.

A very classy 1920s-era bar right in Grand Central Station.

A hands-on traveling collection of the innovative devices of yesteryear.

Elevated freight railway turned wildly successful urban park.

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

A treasure trove of artifacts, books, and artwork from the "golden age" of exploration.

La Monte Young and Marian Zazeela's "Dream House" will immerse you in an ever changing world of sound and light.

A terrific purveyor of natural history objects and curios.

A hidden bathhouse in one of Stockholm's oldest and busiest neighborhoods is open to anyone who can find it.

Perched above the bustling town of Kerak is a sprawling, ruinous castle that has witnessed nearly a millennia of Levantine history.

These ruins are a hidden gem, complete with fantastic views over the cliff edge.

This castle has links to Portugal’s founding and struggles for independence.

Named for the biblical warrior, the largest castle in Israel actually dates to the Middle Ages.

Italian poet Dante Alighieri is entombed in this small mausoleum in Ravenna, and his hometown of Florence has never been too happy about it.