A war-torn Olympic relic is being overtaken by graffiti and nature.

An empty relic of the Moscow Olympics crumbles away in Estonia's capital.

An unassuming secret door once led to a legendary watering hole.

Hidden in plain sight on a pair of Manhattan subway tunnels are a pair of public instruments that sing to each other.

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

An abandoned football stadium with a tragic past.

The best-preserved ancient theater in the world.

An eccentric artist's abode in this small Italian town has been transformed into a surreal temple of mosaic madness.

This famed Silk Road oasis was one of the largest cities in the world before it was destroyed by a Mongol horde.

The highest rapid transit station in the world.

A Mexico City subway station boasts a hidden museum, complete with a working radio booth.

This structure atop a craggy cliff in one of the most beautiful parks in Paris boasts an unbeatable view.

A retired subway car stands alone in a field in the middle of Queens.

Water and windows define this defensive Italian well.

Spanish architects used seized Muslim gravestones to rebuild parts of the fortress.

The first modernist church in Europe.

The housing complex served as an anti-fascist fortress during the Austrian Civil War.

Vienna's diversity-themed traffic lights were inspired by the Eurovision Song Contest.

A history of heat is on display at this unique Austrian museum.

This series of Baroque passageways in Vienna's cultural center mix old architecture with modern art.

Apartment complexes built into huge repurposed natural gas tanks.

The solemn exterior conceals an opulent interior of this 17th century church.

Vienna's underworld of tunnels and subterranean rivers made an iconic appearance in a 1949 Orson Welles film.

A neo-Gothic church built to thank God for saving the Emperor of Austria from an assassination.