The world's longest known cave system.

You can walk through the first Boeing Air Force One, which famously carried JFK's casket from Dallas back to D.C.

Inside houses a vast collection of military aircraft and missiles spanning the ages.

Crystal hunters can dig for their own stones at this DIY mine in Arkansas’ quartz belt.

A city park that sits on top of a freeway.

This downtown Dallas art installation is always on the lookout.

Once the smallest park in the national system.

The largest cattle market on earth holds live auctions open to the public every week.

The Bucyrus-Erie model 1850B electric shovel, or "Big Brutus," the largest existing electric shovel in the world.

The first declared National Monument in the United States.

One of America's first dinosaur parks gives a window into Depression-era paleontology.

One of the first intercontinental ballistic missile sites in the United States.

The granddaddy of all tourist traps, built on ice water, jackalopes, and a giant dinosaur.

Few things look more out of place than the electric blue ponds in the reddish-brown desert of Utah.

The marker offers a rare chance to be in four U.S. states at once, though there's some controversy over its geographic accuracy.

Legends surround this jagged rock formation in the New Mexico desert.

A wacky, UFO-shaped house in a suburban neighborhood.

Hike the ruins of a zoo that was shut down by an unsolved murder.

Welcome to old Florida, where a 1940s mermaid show is still enchanting visitors.

Florida might be at its most surreal in this museum devoted to the famed Spanish artist.

This small museum in a 17th-century building honors three iconic Scottish writers.

A labyrinthine network of chambers with a storied past hides below Edinburgh's South Bridge.

An incarceration museum on the spot of possibly the oldest prison in England.

This one has sailed five times as far as Sir Francis Drake's famous ship.