Travelers exchange letters with woodland creatures at this mini post office.

According to local lore, this narrow passageway through two large rocks was accidentally created by the devil himself.

A beloved carnival gets a modern makeover in Los Angeles.

A retired firefighter created this contraption so he and his family wouldn't die if accidentally buried alive.

A hidden three-foot-wide house separates these two regal churches.

One of New York's best collections of pinball is hidden in a secret bar in the back of this laundromat.

Hidden behind Lincoln's head is a 70-foot-long chamber containing enamel plates documenting American history, sealed in a teakwood box in a titanium vault.

A faux European lane smack dab in downtown L.A.

A painfully obvious spy house sits right across the street from the Russian Embassy.

Daredevil repair workers can worm their way out the access hatch, loop ropes over the apex and rappel down the monument.

A room hidden behind a swinging shelf in the children's section.

The nearly 200-year-old folly has a secret spiral staircase hidden in one of its pillars.

The winding route passes through an enclosed alleyway, two tunnels, and leads to the White House basement.

An unexpected Easter egg lurks in the background of a cherub wall relief at the Breakers mansion.

The enormous 250-room Vanderbilt mansion conceals hidden doors and secret passageways.

The randomness of this wall of lava lamps helps encrypt up to 10 percent of the internet.

Finale of "William Tell Overture" can be heard as drivers pass over grooves in the road.

A black and white painted wall creates an optical illusion in the California desert.

One of the last free public hot springs is decorated with hippie art.

Abandoned bridge hidden deep in the San Gabriel Mountains.

An outdoor sculpture park of giant pink robots and post-apocalyptic Christmas scenes.

The "world's biggest dinosaur" once housed a creationist museum in its abdomen.

Located on 420 acres in the high desert, this spiritual center is dedicated to the "science of the future."

An eclectic trailer park retreat in the middle of the California desert.