This massive subterranean fissure is known as the "underground Grand Canyon."

Mysterious face-like images that appeared within an Andalusian house are surrounded by legends of the paranormal.

The narrow mountain road that carves a path between Florence and Victor is tied to ghost towns and ghost stories.

An unusual slot canyon in the Aztec/Navajo Sandstone has a cross-section like an inverted old-fashioned keyhole.

Once a sewer culvert, this road tunnel now offers cyclists and pedestrians a subterranean stroll under the C&O Canal.

An abandoned railroad tunnel transformed into a creepy path for hikers and cyclists.

The U.K.'s first underground railway line is now a lively thoroughfare for runners and cyclists.

In Hong Kong, a fairytale flower tunnel is hidden within the gardens of a public housing complex.

The romantic botanic tunnel feels like a well-kept secret.

A unique rock formation that only a few have managed to venture inside.

How did a French journalist’s grave become the sexiest statue in the cemetery?

Spectacular mineral formations fill these caves, which have been used for everything from smoking meat to stashing illicit liquor.

This Welsh dockyard was the site of the not-so-secret construction of the Millennium Falcon for the original Star Wars trilogy.

A seaside cemetery honors the Commonwealth soldiers and sailors that died in Sierra Leone during World War I and II.

This garage is frozen in the late 1990s, full of rare and untouched cars and motorcycles.

The only fully-realized skyscraper built by the famed American architect, Frank Lloyd Wright.

This arcade is one of the last places to catch a public performance by the now-defunct ShowBiz Pizza's house band of animatronic animals.

This 20th-century replica of a 13th-century castle has itself fallen into ruin.

The "World's Largest Underground Business Complex," a 55,000,000-square-foot city underneath Missouri.

This magical grotto is adorned with a complex pattern of native seashells, minerals, and fragments of colored glass.

This western-themed amusement park is host to a large number of decaying animatronic cowboys.

This 19th-century religious artwork was once highly controversial—because it was donated to the church by a divorcee.

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

Floor-to-ceiling frescoes by local art students cover the interior of a nearly 200-year-old church.