This private skyway is filled with art and was expanded explicitly so that Hitler could appreciate a river.

These caves secretly held national treasures during World War II.

This mystical orb covered in strange symbols is thought to have been used in ancient Greek magical rituals.

Elaborate stone houses constructed from the ruins of war.

This small chapel holds the mummified heart of one of the heroes of the Greek Revolution.

The railway carts travelers to central Athens's highest peak, which offers stunning vistas of the city.

The Cycladic architecture makes this Athens neighborhood feel like an island paradise hidden in the shadow of the Acropolis.

The site where hundreds of Greeks were tortured by the Nazi secret police is now a cosmetics store.

Where commuters walk through remnants of an ancient Greek necropolis.

This abandoned shopping arcade has new life as an art installation.

The ruins of an ancient Roman tomb hidden in the basement of a trendy clothing store.

One of the best collections of Byzantine art housed in a magnificent, 19th-century neoclassical home.

This coffee shop lies hidden beneath a historic church.

One of America's founding fathers lived in this London lodging between 1757 and 1775.

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

A charming 17th-century alley is lined with secondhand bookstores and antiquarian shops.

This third-generation institution is one of the last in London that still cures its own salt beef for epic sandwiches.

A commemorative plaque at the door pays tribute to an "American Patriot" known for his betrayal.

Legend says these colorful, invasive residents are the descendants of birds released by Jimi Hendrix.

This London marker celebrates the spot where the world-famous English band was founded.

This London plaque marks the spot where David Bowie's famous alien made his debut.

Never actually a tree at all, this spot was the site of London's public hangings for nearly 600 years.

The wooden telephone box hidden away in London's Piccadilly was the original prototype of the iconic kiosk.

London's last remaining public soapbox site has seen speeches from Karl Marx, Vladmir Lenin, and George Orwell.