A hidden network of subterranean passageways lies tangled beneath Indianapolis' bustling City Market.

Whisper secrets in a busy station only to be heard on the other side of this acoustic arch.

These delicate buggers are some of the rarest fossils that have bubbled up from the Tar Pits.

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

A walk-in working camera obscura produces 360-degree live images of the San Francisco coast.

This knobby, drooping New Orleans oak is a favorite place both for climbing and for meeting giraffes.

The longest continuous lava tube in the continental U.S. offers 2.5 miles of dark explorations for the adventurous.

A modernist landscape of water and sci-fi concrete.

A huge musical sculpture and other art installations on the National Oceanic and Atmospheric Administration grounds.

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

A stunning, two-tiered waterfall made famous by poets, artists and the legend of a ghost dog.

Established in the '60s, a family-owned tropical drink bar founded by Ray Buhen, bartender to the stars.

All that's left of the quintessential "cabin in the woods."

This 4,000-foot-diameter hole is touted as "the most well known, best preserved meteorite crater on Earth."

The world's longest known cave system.

One person's lost luggage is another person's treasure.

This geothermal hot spring hides beneath a slowly growing mineral dome.

This natural wonder drops 16 stories straight down through rare flora and streaming waterfalls.

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

One of the oldest used bookstores in the U.S. has been selling antiquarian treasures since 1825.

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

Read a book next to a Midtown waterfall.

A heavenly experience, right here on Terra Firma.

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