An arsenic-filled pool in the Grand Canyon takes on the appearance of a giant, overflowing pumpkin.

This secluded aqua waterfall in the Grand Canyon is the perfect swimming hole, and the Havasupai tribe's fiercely protected natural wonder.

This ancient cave is home to a museum, bar, and is filled with fluorescent minerals.

The hidden offices underneath Memorial Bridge have been locked up since 1976.

The Byzantine, pre-Columbian, and medieval art at this stately mansion are some of the most under-appreciated collections in D.C.

The country's oldest continually-operated mini golf course lies hidden in plain sight, eclipsed by one of D.C.'s most popular tourist attractions.

The sci-fi villain is a little-known inhabitant of the U.S. capital's largest cathedral.

A tiny piece of the Moon is embedded in this stained glass masterpiece.

This grand gothic brewery has been pumping out suds for over a hundred years.

A cavernous three-story, 43,800-square-foot basement that was forgotten about for 60 years.

Franciscan monks created a facsimile of the Holy Land for North Americans who couldn’t afford the trip overseas.

Home to items never before seen by the public.

With over 100 jam-packed rooms to explore plus elaborate tea services and events, the Mansion on O Street is a hidden treasure.

The site of the climactic scene from the classic horror film is now a historic landmark.

Long-abandoned trolley tunnels just a mile away from the White House are turning into an art space.

The flower-shaped temple in Virginia aims to transcend the boundaries of the world’s religions.

This maze-like slot canyon is a unique gem within an already fascinating geological area.

A railroad tunnel that, upon its completion, was considered one of the engineering marvels of the modern world.

Outsider art meets paleontology at this roadside reptile repository.

A 300-yard long natural tunnel runs through a ridge deep in the Appalachian Mountains.

Giant busts of 42 U.S. presidents are sitting in a field in Virginia.