The remains of Crystal Palace Great Exhibition of 1851.

The world's first underground farm is in a World War II bomb shelter beneath the streets of London.

An alleyway plaque is all that remains of the independent Texas' envoy that forgot to pay their rent for over a century.

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

In this cobwebbed old operating theater, you can practically hear the screams of the unanesthetized patients.

Rebuilt remains of a temple to Roman god Mithras.

A special shop catering to the needs of things that go bump in the night, and also helps kids become writers.

The signs of a long buried road can still be found hidden beneath a London sewer grate.

A vast overgrown cemetery, once known as one of London's "Magnificent Seven."

The curios of pharmacist and collector Henry Wellcome.

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

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

Winding paths and ramshackle mausoleums in the oldest "park cemetery" in London.

A kaleidoscopic warehouse-maze of handmade neon signs.

A 300-year-old tea shop that brought tea to the English people, not to mention the Queen herself.

Hidden beneath the streets of London is a secret subway that only carried mail.

London's creepiest cemetery was once the site of dueling magicians and mobs of stake-carrying vampire hunters.

Visit the ancient, mysterious Roman catacombs ... in the Netherlands.

This historical Netherlands fortress still bears the marks of its single siege.

Incredible labyrinth of man made caves.

This centuries-old gothic monster has been converted into a sprawling, glossy bookstore.

What was once a bustling mining installation is now a bustling hub of contemporary art.

Remarkably preserved body discovered in a peat bog.

A 14th-century fortified keep showcases the masterpieces of an ingenious 20th-century clockmaker.