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

Hanging over one of Japan's "hidden valleys," these vine bridges must be remade every three years.

The abandoned Japanese amusement park once loomed as a dystopian Disneyland.

Experience park where visitors are told to expect the unexpected.

Shrine to Japanese water-goblins, complete with their preserved body parts.

Hot springs monkey spa in the middle of Hell's Valley.

Deep under Manhattan is the largest construction project in city history. So far, the project has taken the lives of 23 workers, or "a man a mile."

The world's largest mountain carving could fit all of Mount Rushmore inside it many times over.

Twenty-seven massive radio antennas on the high plains of New Mexico search for life on other planets.

A replica that won't float away.

Most-visited and most-photographed slot canyon in the American Southwest.

The ancient dwellings of the Puebloans, located in the cliffs of majestic mesas.

The largest self-built castle in the U.S., six decades in the making.

Treehouse motel and treehouse-building school.

World's only nuclear reactor operated by liberal arts undergraduates.

Beyond being incredibly fun, not much can be said about the Hole with certitude.

A volcanic fissure over two miles long and up to 70 feet deep.

Another in a long tradition of legends regarding infernal seats.

Patrick Dougherty's fantastic living tree sculptures, mind-blowing but temporary.

A late sculptor's last project was to turn this abandoned cement factory into a castle-themed amusement park.