This groundbreaking 16th-century map is known as "America's birth certificate."

The hidden offices underneath Memorial Bridge have been locked up since 1976.

There's a 12-foot-tall replica of the Washington Monument hidden under a manhole nearby.

This mysterious structure by the Washington Monument is a flood barrier designed to protect the White House against rising waters.

An overlooked memorial honoring the local Washington residents who died in World War I.

This unassuming alcove in London’s famous Tower Bridge once served to capture, store, and display dead bodies from the River Thames.

On the grounds of this study, Lew Wallace composed "Ben-Hur: A Tale of the Christ" and many other works.

Once the smallest park in the national system.

This national park on the shore of Lake Michigan features remarkable dunes that lie at the heart of a Chippewa legend.

These ruins of a pre-Columbian cliff village are among the most beautiful and well preserved in New Mexico.

A 19th-century cabin built into a hollowed-out Sequoia tree.

At the top of the hour, this steam-powered clock's eight brass whistles play "Back Home Again in Indiana."

Rome’s legendary peephole vista is also a former Crusader stronghold.

The remains of a World War II-era Japanese American internment camp dot a windswept Colorado hill.

Once a lonely sentinel on the edge of the Great Plains, this rebuilt trading fort offers a glimpse of life on the Santa Fe Trail.

The world's largest anatomically accurate brain sculpture.

This giant pocket watch is the third iteration of a local landmark that has been standing just off the Santa Fe Plaza for over 100 years.

This innocuous New Mexico storefront was once the secret jump-off spot for Manhattan Project scientists.

The hallowed remains of a 900-year old Ancestral Pueblo great house.

This popular pitstop lives up to its name.

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

Twenty-seven massive radio antennas on the high plains of New Mexico search for life on other planets.

The ruins of this former mining boomtown once housed over 3,000 people. Now, the population is two.

Thousands of pieces of art have been carved into the rock at the site of an ancient volcanic eruption.