This Nazi-built amphitheater was constructed on a sacred mountain site used by various German cults.

This 220-year-old house is a reminder of New York City's Dutch past.

A 16th-century Andalusian palace.

Spain's unique Mosque-Cathedral has been a place of worship for both Muslims and Christians throughout its history.

The humble fort that saved Washington.

Sitting in the basement of a castle, the largest wine barrel in the world is bigger than most apartments.

Vienna's giant wonder wheel had a starring role in one of the greatest noir movies of all time.

Rusting away on the edge of New Jersey are the remains of a Cold War missile launch site.

A little-known subterranean street runs under some of the best-known buildings in Chicago's River North.

The perfectly preserved underground rooms where Churchill plotted the war against Germany.

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

The comedy and tragedy of Oscar Wilde is preserved in a Dublin park, captured in a sculpture of colorful stone.

Museum devoted to the artificial language of Esperanto.

Handsomely crafted representations of our round planet.

A patriotic medieval knight in driven-nail armor raised money for widows and orphans of WWI.

Once America's longest-manned military fort, now an abandoned hulk.

This Delaware River bridge is emblazoned with a catty slogan from a more prosperous time.

Theme park featuring miniature replicas of European monuments.

A little sculpture with a large wardrobe.

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

Climb inside an iron crystal magnified 165 billion times its normal size.

Ancient serpentine sacrifice holder that has stood the test of time.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

The bunker where Hitler took his own life is hidden under this deliberately ordinary parking lot.