A 1,215-foot tunnel transports pedestrians beneath the River Thames.

Amazing and mysterious Neolithic settlement on Scotland's Orkney Islands.

This medieval castle overlooking the Elzbach river looks just like it did hundreds of years ago.

Perfectly preserved ancient monastery in an impossibly dramatic location on a rocky island in the Atlantic.

This semi-secret storybook hedge maze is also the oldest garden in the city.

The hidden lair of an alchemist rediscovered in a 2002 flood.

The birthplace of an eccentric alchemist and possible inspiration for the fictional reanimator of monsters.

The intricate stonework of Machu Picchu serves as a testament to Incan engineers.

The history of occult science is turned into a creepshow at this lurid Prague tourist attraction.

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

A Hungarian pharmacy museum with a box full of mummy powder.

Sitting beneath an unremarkable residential building is a museum devoted to classic Pinball.

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

A historic cave in Scotland once served as a hiding place for a famous Covenanter.

A mysterious monument where a woman who records say never existed was burnt alive for being a witch.

This ethereal, rainforest-like gorge is home to waterfalls, quaint bridges, and fairy folk.

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

Overlooking a Scottish loch, this beautifully crumbling castle once inspired a poem by William Wordsworth.

This ancient church, rebuilt at least 3 times, is flanked at all sides by history.

A mysterious pair of mice eating a piece of cheese is London's smallest public sculpture.

Rebuilt remains of a temple to Roman god Mithras.

Remnants of cells from the most notorious London prison may lie in the basement of a rare Victorian Gin Palace.

A subterranean ice warehouse that has lain hidden beneath central London for over a century is now open to the public.

This macabre monument atop an ancient well gives a nod to a brutal episode in Scottish clan history.