Disks are rolled out onto the south lawn to absorb the impact of Marine One's wheels like giant coffee coasters.

Before Metro, Washington had a robust streetcar network—and you see the remains of this infrastructure if you know where to look.

This unique historical monument features large concrete spheres designed to resemble fallen stars.

This twisty sign depicts the chain's first logo.

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

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 remarkable feat of Victorian engineering across the Firth of Forth.

An echo chamber with a waterfall wrapped around its base at the Embassy of Canada in Washington, D.C.

The Aldie Gristmill was once the largest factory of its kind in Loudoun County.

One of the two original gateways into the Kasbah, this gateway is a nesting site for storks.

This twice-relocated "Usonian" home is among the smallest built by master architect Frank Lloyd Wright.

Architecture lovers won’t stop touching the National Gallery's 19.5 degree marble prow.

The actual model from the original "Star Trek" series is now on display at the National Air and Space Museum.

While most wind tunnels test scale models, the "Cave of Winds" was large enough for actual airplanes.

The oldest continuously operating fish market in the United States.

This sandwich shop has a century-old elevator behind a sheet of plexiglass.

D.C.'s first central library was born out of a chance encounter with the philanthropist whose name it bears.

Daredevil repair workers can worm their way out the access hatch, loop ropes over the apex and rappel down the monument.

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

The oldest building in the District of Columbia was preserved because of a mistaken connection to George Washington.

The fairy tale castle of the "fairy-tale king."

This abandoned mine offers a glimpse at what life was like for those who came to Alaska seeking Gold Rush riches.

These unusual rooms on wheels are holdovers from the 1960s.

The only remaining African American schoolhouse in northern Virginia is now a museum dedicated to formerly enslaved people.