The first tunnel to be dug beneath a river, repurposed as an art and museum space.

The "father of immunology" used this humble hut to offer the local people some of the world's first free vaccinations.

This Budapest bar allows drinkers to leave their mark on the walls, floor, and even ceiling.

This unattributed sculpture in Amsterdam is hiding in plain sight.

A unique collection of gilded age mechanical pianos.

The Bloemen Route, or “Flower Route”, stretches 25 miles through prime Dutch tulip country as a result of “tulip mania,” the world’s first economic bubble.

Tucked away in the Leiden city hall are a group of works by famed artist MC Escher.

Peek through a transparent pyramid and drink in the mountain made of books.

One of the oldest libraries in England, and one of the last of the great medieval chained collections.

This collection of unique liquids allows visitors to wander the waters of Iceland's glaciers.

Hidden away in a modern library are the remains of one of the 18th century's most celebrated wunderkammers.

A free library to stumble across, one of many placed worldwide if you know where to look.

A three-shelf library serves as a memorial in the Jewish cemetery of Krems.

A 19th century artistocrat's mansion, turned into a library, hidden in a modern library.

This everlasting lightning storm may be the world's largest generator of ozone.

The world's largest collection of intentionally useless inventions.

The largest ice cave in the world is thought to be the entrance to hell according to Austrian legend.

Trick water gardens from 1619 featuring a 200 piece mechanical theater powered by water.

Vaucanson's most famous automaton, brought back to life.