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

The world's largest man-made underground caverns offer unparalleled underwater wonders.

A museum dedicated to man's boldest venture.

Some of the oldest exposed rock formations in the United States.

A natural gas leak provides a flickering flame beneath a waterfall.

The overgrown remains of an old rail bridge rise above the valley like ancient Greek ruins.

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

Crystal clear lake with a shoreline of travertine located in a Colorado canyon.

This hill made entirely of ancient Roman jars was simply a garbage dump to the people of the time.

Visit 17th-century ruins on a strikingly beautiful riverside walking trail.

Historic, 19th-century wagon wheel ruts cut into soft volcanic ash.

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

Naturally formed white marble arch housed in an abandoned marble quarry from the 1800s.

An LED art installation composed of 600 poles wrapped in kimono fabric.

Few of the many visitors to this resort town know of its hidden Maya ruins.

An obscure waterfall that looks like something out of a fantasy novel.

One of the American Southwest's most extensive collections of Navajo rock art.

Seemingly grown on some other world, these New Mexico rock formations look like a disused science fiction set.

These otherworldly limestone formations jut out from the Kansas prairie.

George Washington's dentures and Queen Victoria's dental instruments are in the nation’s official museum of the dental profession.

One of the oldest anatomical theaters in the U.S. still in use was once the site of an infamous cadaver riot.

The last manually operated turntable ferry in the world.

Read between the lines of the subway map to find the hidden abandoned Civil War fort.

Where manatees are the snowbird vacationers in these balmy Florida springs.