100 years later, they're still down there.

Now a living classroom, this house was once the home of Reverend James Clinton Dent, a formerly enslaved man and a pastor.

This small pedestal marks the spot of the first lot of land sold in Alexandria, Virginia in 1749.

This cannon dating to 1755 has stood guard at this suburban intersection since 1915.

Long buried under the 20th-century landscape, this lift lock of the Alexandria Canal is the lone remnant of an ambitious early American transportation project.

A small and somewhat hidden memorial to 68 local veterans of the Vietnam War.

A portal into the bricked up canal that runs through the heart of Washington D.C.

This apartment building designed by a notable Washington D.C. architect housed the Qing Dynasty's Legation.

This historic boathouse was constructed using salvaged lumber from burned barns.

A hoard of sky catalogs, astrophysical journals, even the works of Galileo and Copernicus.

A building shaped like the Army Corps of Engineers logo, because why not?

Surrounded by traditional homes, this Tolkien-esque dwelling stands out on a quiet street in suburban Maryland.

The headquarters of the Audubon Naturalist Society is a small slice of heaven for nature lovers.

This small cemetery remains hidden behind a recreation center.

The original movie set for the home of 'Star Wars' hero Luke Skywalker.

The world's largest prehistoric copper mine had 30,000 animal bones scattered throughout the maze of passageways.

Spend the night in this formerly abandoned traditional Scottish village.

The air force base preserves its scars from the Japanese attack on Pearl Harbor.

Hiding in plane sight.

Running since 1864, this clock has yet to be wound, one of the longest-running science experiments known.

An authentic, seaworthy replica of the ship that sailed to Newfoundland in 1497.

Passengers are not allowed to set foot outside this station, but you can still enjoy spectacular views of the sea from the platform.

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

This statue honoring the most decorated dog of World War I stands in a Connecticut park.