Rebuilt remains of a temple to Roman god Mithras.

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

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

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

One part local history, 99 parts gruesome haunted house.

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

Eighty million natural history specimens call this gargantuan museum home.

The best way to get a sneak peek at the movies.

Whisper secrets in a busy station only to be heard on the other side of this acoustic arch.

The remains of a historic football stadium that once hosted 50,000 spectators.

These ornate arches show how the course of the River Thames has changed.

An eccentric 18th century naval officer built a ship's quarterdeck on his roof, the inspiration for Mary Poppins's "Admiral Boom."

19th century tunnels burrowed under Liverpool.

A unique set of caves that spits waves back out into the ocean.

Climbing this abandoned railway leads to breathtaking vistas of O‘ahu.

The ruins of Vancouver's abandoned heritage railway.

At just six feet two inches deep, it is said by Guiness and Ripley to be the world’s narrowest freestanding office building.

A working steam clock, one of only a few in the world, located in Vancouver's Victorian Gastown.

What remains of the summer palace of King Kamehameha III.

Formerly known as Saville Theatre, this mid-20th-century entertainment hub includes an ornate frieze.

Chelsea's soldiers were given strict orders when crossing this historic bridge.

The bronze uniforms commemorate the vital role of women during World War II.

Sculptor Antony Gormley used chaos theory and fractal growth to design his tallest creation.

The pretty iron lamp is a reminder of the architecture underneath London's streets.