An underground city of ancient quarry tunnels hid thousands of Allied soldiers in WWI.

This triumphal arch celebrating the Sun King's conquest of Lille is a masterpiece of military architecture.

This monumental mural crafted nearly 40 years ago is still in perfect condition.

An unsettling sculpture of a boy hanging out in a street in Ghent has been baffling visitors for years.

This narrow Ghent alleyway is an officially sanctioned street art tunnel.

Eighty million natural history specimens call this gargantuan museum home.

This tree was grafted from the actual tree that led Isaac Newton to ponder the theory of gravity.

Named after a lodging site that existed nearby, a bridge has existed at this location since 1455.

A giant wooden soda bottle pays homage to a distinctly British drink on the very spot where it was invented.

Have a seat with the father of modern computing at this Manchester monument.

After protestors toppled a statue of a 17th-century slave trader, the pedestal where it once stood remains unoccupied.

An ornate doorway leads to the former site of one of the city's oldest society of merchants.

This Grade II Listed Building with an Art Nouveau façade once contained a unique print shop.

17th-century merchants in Bristol nailed their deals by placing money on these pedestals.

The unusual clock on this 18th-century market building has an extra minute hand for "Bristol time."

Designed to celebrate the queen's 40th birthday.

A miniature replica of the canine that became the logo for a large recording company.

Banksy's image of a naked man dangling from the window of a sex health clinic.

A 142-foot-tall Q-tip on Bristol's skyline.

This church is said to have connections to the Knights Templars.

An alchemical monument that represents "Earth, Air, Fire, and Water."

There's a labyrinth of manmade tunnels stretching for over an acre beneath Bristol.

The unpredictability of this pendulum's movements has drawn visitors to this magnificent church for decades.

A 15th-century whalebone is tucked away in an unassuming corner of this medieval church.