An 18th century fountain and two columns are all that remains of Rome's forgotten river harbor.

This one-of-a-kind Roman arch is not actually dedicated to the god Janus.

This gallery commissioned by Cardinal Spada is known for its magnificent optical illusion.

This church is named for one of the most dramatic tales in the early Christian church.

This 50-foot-deep sinkhole created in the 14th century is home to rare plants and fungi.

Many famous English poets including Elizabeth Barrett Browning are laid to rest at this Swiss-owned cemetery in Florence.

This museum housed in a former brothel explores the past and present of sex work in Amsterdam.

A curious collection dedicated to human and animal anatomy and pathology.

These temples are among the oldest free-standing stone buildings in the world.

Macabre astrological automaton clock dating to the late 1400s.

An unusual round church in London with a Templar past.

This towering church is home to a world-famous organ and a treasure trove of medieval art.

A 13th-century convent where St. Margaret of Hungary spent her teenage years.

A two-way chairlift system between Zugliget and the highest point in the city.

Installed in an act of one-upsmanship, the annual sundial in this church is also a symbolic victory for the Gregorian calendar.

The best preserved Paleochristian basilica in Rome hides a number of oddities and secrets.

A huge enchanting public park just outside the ancient walls of Rome.

A detailed pachyderm supports Rome's smallest Egyptian obelisk.

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

A centuries-old forgotten underworld lies beneath Malta.

These lesser-known Maltese catacombs date back over 1,700 years.

This spectacular dome managed to survive after being bombed in World War II, in an event known as the "Miracle of Mosta."

This spaceship sculpture in the middle of the Slovakian capital looks like it just crash-landed.

Time stands still at this serene burial ground in the center of Budapest.