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

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

The world's longest known cave system.

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

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

A "gravitational anomaly" located in the redwood forests just outside of Santa Cruz, California.

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

A favorite haunt of dinosaur loving children and hallucinogen loving hippies alike.

The inspiration for H.P. Lovecraft's horror stories and Batman's Arkham Asylum is now a condominium complex.

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

A strange house built only at night, with rumored mystic inspiration.

Towering basalt columns, a creation of fire and ice.

A beautiful geyser under environmental threat.

A technicolor place of worship for Elevationists.

This abandoned amusement park was built on a site with a dark history.

A hand-built folk art installation covered with messages of God's love.

An emporium for the weird, creepy, rare, and just plain gross.

The first declared National Monument in the United States.

Eighteen miniature death-scene dioramas.

The rocks in this arid basin leave trails of movement behind them.

The only theatre in the world constructed specifically for sound movement.

Home of an evil witch famous within southern folklore.

The remains of a failed attempt at medieval nerdery are hidden deep in the woods of Virginia.

The deepest lake in the United States, and once the site of epic destruction that lives on in legends.