This former Nazi castle was built as a school to train what would have been the future of the Third Reich.

This mysterious pig iron pillar does not rust, despite being weathered by the elements for centuries.

The largest mosque in Germany.

This mine is home to the largest man-made hole in Europe.

This concrete church spent a period of its history as a World War II bunker.

This highrise residential building looks mundane, but hides a water tower and WWII bunker.

No need to strap in for a "ride" on this walkable roller coaster.

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

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

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

Wedged between two buildings at the Great Ormond Street Hospital is this secret music installation.

An easily overlooked stretch of ornate window fronts and gaslight lamps that could be right from the pages of Dickens.

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

This London pub hides a peculiar secret: a recreation of the rooms shared at 221b Baker Street by Sherlock Holmes and Dr. Watson.

The popularity of Sherlock Holmes led to the creation of his fictional address, turning the orderly London street numbers askew.

This ornate Victorian marketplace was the setting for Diagon Alley and the Leaky Cauldron in the Harry Potter films.