A monument celebrating the idea of reaching inaccessible places.

The world's second largest single-dish radio telescope, once used to contact E.T.

Commercial airplanes land mere feet over the heads of sunbathing beachgoers on this tiny island.

A city continually buried under the ash of a volcano that has been erupting for more than 15 years.

The youngest volcano in Central America.

A massive underwater sinkhole made famous by Jacques Cousteau.

No telescopes, no power, no gift shop: The Observatory of Chichen Itza, one of the world's oldest observatories, takes astronomy back to basics.

From overly ambitious (and eventually abandoned) atomic dream to bustling office park.

Clap your hands at the base of the pyramid, and the song of a sacred Mayan bird will echo through the air.

A smoke stack so large, the Washington Monument could fit inside!

New fungal and bacterial species call this deadly lake home.

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

A vertically floating hemlock controls the waves and weather at Crater Lake.

All aboard the world's longest miniature railway.

A working model of the entire San Francisco Bay and much of the Sacramento River Delta, complete with regular tides that change every 14 minutes.

Guns that have been defending San Francisco since 1904.

A huge musical instrument played by the ocean.

A collection of 20th-century automata, penny arcade games, and musical contraptions.

Not merely a museum – but a whirring powerhouse that offers a subterranean peek into the heart and soul of cable car operations.

The White House Situation Room was taken apart in 2006 and shipped to the Ronald Reagan Presidential Library.

An old saloon in the West filled with history, and according to some, a few dozen ghosts.

The world's largest rose bush has literally outgrown the pioneer museum it was planted behind.

This giant steer-skull edifice refuses to die.

A one-of-a-kind sundial at the Very Large Array honors a radio astronomy pioneer, incorporating the original piers from his breakthrough radio telescope.