A 140-year-old castle with underground caves hidden in San Francisco.

It should come as no surprise that the best view of the Hollywood sign is from Batman's lair.

High on a hill overlooking Los Angeles lives a world famous, high power conical Tesla Coil.

The legendary building that needed ghostly approval before being built.

In all of the 29 years it was in operation, no prisoners ever escaped successfully... or so they claim.

Bring your own cardboard to the slippery slopes of concrete hidden in a neighborhood park.

This iconic L.A. bookshop is housed in an abandoned bank—both symbolic and chic.

An artsy hidden staircase leads to breathtaking views of San Francisco.

A huge musical instrument played by the ocean.

Once home to a local bear named "Bruno."

A small plaque highlights the damage caused by German air raids during World War II.

This hidden museum examines the complex history of the country's law enforcement.

Belfast's namesake river still winds below the city center, hidden from sight.

A palatial former bank building now houses what may be the poshest art venue (and former Tesco) in Belfast.

The ship built to ferry passengers and goods to the doomed Titanic is now the last remaining White Star Line vessel.

The statue honors a beloved bird that once roamed St. George's Market.

These barriers stand as stark reminders of Northern Ireland's troubled past.

The only monument to name all of the passengers and crew who died on the RMS Titanic.

The last Victorian market in Belfast.

An enigmatic monument to the Irish revolutionary who plotted an uprising against British rule.

An 80-year-old plane tree is devouring an iron bench at Ireland's oldest law school.

A guitar mounted on a wall in Temple Bar honors the Irish musician.

Amidst the bustling streets of Dublin, there is a hushed garden that is dedicated to "All those who gave their lives in the cause of Irish Freedom."

Oldest building in California still in use.