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

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

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

A modernist landscape of water and sci-fi concrete.

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

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

The world's longest known cave system.

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

Two thousand artifacts from around the world collected by one woman who loved to travel.

This naturally-sourced hot spring is now home to a clothing-optional resort.

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

One of only two commuter aerial tramways in the United States.

A self-built castle made by a mysterious man.

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

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

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

This frequently-photographed waterfall takes only a short hike to visit.

Take a drunken tour inside the world's largest known geode.

Hidden in Central Park's Ramble is a troublesome cave buried just beneath the surface.

Ninety feet off the valley floor is an ancient dwelling put under protection of the U.S. government by Teddy Roosevelt.

A beautiful geyser under environmental threat.

A dilapidated dock tells the story of Titanic's missed arrival.

The hike through a narrow river flanked by the 2,000-foot-tall walls of Zion Canyon is one of the best in the world.

Tour the dark and beloved poet's former cobwebbed basement, which may have inspired "The Black Cat."