The largest underground pedestrian tunnel system in the U.S. mostly caters to the people working in the offices above, but provides a great, air-conditioned way to traverse the city.

One of the world's largest telescopes, stares into the night sky with two huge mirrors.

The biggest solar telescope in the world helps reveal the secrets of our sun.

A planet-hunting astrograph built in the 1920s to seek out the so-called "Planet X" beyond Uranus.

A pair of large white marble lions guard this stretch of the Mojave Desert, and it is utterly mysterious how they got there.

Artist Jack Pierson made this surreal sculpture in honor of Wonder Valley, a countercultural community on edge of civilization.

A monument commemorating the discoverer of radio astronomy.

A giant statue perched on dumpsters watches over the freeway.

An abandoned military bunker that has been transformed into an artist's paradise.

This museum-within-a-museum pays tribute to "The World's Most Experienced Airline."

The stone marker that Mason and Dixon placed at the eastern end of the famous north-south border they surveyed.

A rare wayfinding portmanteau of “Street” and “Avenue.”

At the point of oceanic inaccessibility, you can't get farther from dry land without leaving the planet.

This nondescript tunnel has been featured in a number of iconic movies, from “Roger Rabbit” to “Back to the Future.”

Walk in the footsteps of the famed astronaut at Purdue University.

Gold, silver, copper, lead, and zinc were once extracted at this abandoned mine near Lookout Mountain.

For decades, the largest tungsten producer in the U.S. was located in a spectacular setting in the high Sierra Nevada, with some of the workings over 11,000 feet.

Pennsylvanian mass grave and possibly mass murder.

A converted hangar displays dozens of helicopters, from a famous 1943 rescue chopper to several autogyros.

An assortment of armored tanks and Howitzer cannons in the middle of the Sonoran Desert.

This unassuming building houses one of the world's most extensive collections of cars created by automotive legend Carroll Shelby.

The first patented shaved-ice machine heralded a snowy wonderland of iced treats at a favorite New Orleans summer shack.

This Birdsill Holly fire hydrant is the remnant of a very different type of hydration innovation.

This macabre shop, filled with art, oddities, and taxidermy is hidden in the heart of the French Quarter.