The sun acts as a key that only fits this giant keyhole twice a year.

Barely visible foundations remain where Stow Lake’s “observatory” once welcomed visitors.

Floating man-made island and upscale restaurant.

A pair of slides and a tree-lined stair corridor have been an urban oasis for nearly 40 years.

A large instrument played by the wind.

America's most famous piece of self-built architecture.

This strange seaside tower got its name from a wealthy eccentric who would hide coins among its stones.

The old Niles Canyon Aqueduct in the San Francisco Bay Area is a bemusing and secret trail through nature for wayward travelers.

This abandoned subway station beneath the Waldorf-Astoria hotel is allegedly used to secretly transport presidents.

A roaring, awe-inspiring double cascade of icy water flows through woodland Oregon like something out of a Tolkien novel.

An old vending machine serves up unpredictable beverages from its seemingly magical insides.

The abandoned zoo enclosures offer a perfect place to experience the other side of the bars.

A snapshot of the city's fascinating voodoo culture.

A 16th-century horror show built in a lovely Italian garden.

For two weeks every June, fireflies in the Smoky Mountains perform an astonishing light show.

A one-of-a-kind collection of a unique medium.

Dodge prohibitionists and slide through the telephone booth in Crif Dogs.

999 stairs lead visitors through a massive hole in a mountain known as the Gateway to Heaven.

Officially named the "Loneliest Road in America," this barren stretch of Nevada road offers almost nothing but sky and open fields.

This whimsical seashell-shaped place of worship was designed to blend into its surroundings.

Trails through lava-carved tube-caves display sparkling bacteria and a rich history.

Canyon walls shrouded in ferns.

A trash dump made beautiful by nature's power.

An underground mansion with fruit-bearing trees and grapes growing beneath the surface of central California.