One of the American Southwest's most extensive collections of Navajo rock art.

A phenomenal assembly of pueblos in New Mexico is the most complete example of ancient ruins north of the border.

A well-preserved Ancestral Puebloan ruin tucked beneath a desert cliff.

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

A strange sandstone formation in the middle of nowhere presents an interesting climbing opportunity.

The highest peak in the Jemez Mountains and the traditional center of the Pueblo world.

These strange carved boulders in a forested canyon resemble tiny stone villages.

Despite monuments in other locations, this spot in a South Dakota field is recognized by the U.S. National Geodetic Survey as the center of the 50 states.

A group of college students hauled a desk up a mountain more than 40 years ago. The notebooks inside have become a makeshift time capsule.

The discovery site of the "Capitalsaurus," the official dinosaur of Washington, D.C.

One of the largest artillery pieces in the world sits in a Washington D.C. parking lot.

An Industrial Revolution-era public work that purified water using nothing but sand.

The winding route passes through an enclosed alleyway, two tunnels, and leads to the White House basement.

Largest Roman Catholic church in North America.

People escaping slavery may have hidden inside a corpse vault.

A derelict bit of transportation infrastructure hidden in the woods.

A painfully obvious spy house sits right across the street from the Russian Embassy.

Cold War intelligence agents monitored communist embassies from an attic in a former pigeon coop.

A hidden gem on the lawn of the U.S. Capitol Building designed by landscape architect Frederick Law Olmsted.

The United States Capitol's former columns still stand.

A clone of a cloned tree that was so important it allegedly had its own guards.

Enormous piles of historically significant stones, dumped by Congress in a forest, and abandoned for 60 years.

This grand gothic brewery has been pumping out suds for over a hundred years.

Long-abandoned trolley tunnels just a mile away from the White House are turning into an art space.