Remains of Roman amphitheater hidden in a sleepy quarter of Paris.

The vast, legendary catacombs hold secrets much stranger than stacked bones.

The famous winding walk at Chartres is surrounded in mystery and myth.

This whimsical castle of grottoes was the labor of love of a French postman, and one of the greatest achievements of outsider architecture.

Beautiful carved tunnels.

One of the greatest maze designer’s greatest mazes.

Figures of unknown origin carved into the surfaces of underground caves.

A tiled, multi-level garden of survival in Lyon.

The largest underground church in Europe.

This 1,300 year old monastery built atop a single rock was once only accessible depending on the whims of the tide.

An eccentric garden of sculptures crafted in India decorate an Irish green space dedicated to Alan Turing.

The hidden "ballroom" under a mansion lake was built by a criminal who ate a cyanide pill during his trial.

Europe's largest prehistoric mound – burial site of a legendary king?

An intricately decorated tiny place of worship.

A magical place that lets you play in and explore the world of Studio Ghibli.

This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

A 30-acre garden inspired by the principles of modern physics.

A 18th century graveyard and its astonishing cortege of macabre iconography.

A clock designed to run with perfect accuracy for 10,000 years.

A historic arboretum and non-conformist, non-denominational park cemetery.

The largest of London's subterranean rivers and once a mad, bad center of London life.

A house of bones — but nobody is quite sure to whom they belong.

Shakespeare, David Bowie, Dr. Who, the Saxons, Druids and Romans are all claimed to have all made their mark on these massive man made tunnels.

Dioramas and animatronics lead you through the story of Bhagavad-gita, the Indian literary classic.