A mysterious castle built as a monument to lost love.

Frank Lloyd Wright's most iconic home dangles over a Pennsylvania waterfall.

A sacred site for Native Americans surveyed by George Washington and owned by both King George III and Thomas Jefferson.

The world's longest known cave system.

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

A museum dedicated to the alleged Roswell Incident.

The second-largest cave chamber in the world was discovered in 1898 by a 16-year-old and a friend known as "Pothead."

The largest hot spring in the United States is, as the name suggests, a stunning show of natural color.

A beautiful geyser under environmental threat.

The first declared National Monument in the United States.

Possibly the most beautiful carillon in the world, set atop Iron Mountain in Florida.

Enormous lake at the bottom of a unique cave system.

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