The largest mosque in Germany.

The largest bell in the world has been swinging high above a Cologne cathedral for almost a century.

Thousands of tourists and residents in Cologne affix "love padlocks" on the Hohenzollernbrücke Bridge to symbolize their affections.

Golden cobblestones remember a time when this small alley was used to avoid having to give a Nazi salute.

A small square plaque marks a failed assassination attempt that could have changed history.

One of the most easily accessible examples of the rare 1960s UFO houses.

A fearsome dragon clings to a corner of Munich's city hall.

Go surfing on this urban wave, "hacked" by local surfers.

A secret greenspace hides atop a Capitol parking garage.

The 5.5-mile ring of steel posts around the Capitol Building is one of the largest (and most uniform) of its kind in the world.

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

A 1,100-ton train fell through the floor in 1953. Workers got it patched up in just 72 hours.

Home to items never before seen by the public.

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

One of the dwarven trees dates back to 1625 and survived the Hiroshima bombing.

Home to a vast and influential collection of Shakespeareana.

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

The preamble to the U.S. Constitution written entirely from vanity license plates hangs in the Smithsonian museum.

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

The little-known, 300-year history of the area includes former lives as a bustling tobacco port, parking lot, and industrial dump.

The fountains of the U.S. Navy Memorial are annually salted with water from the Seven Seas as part of a nautical ritual.

When it comes to handsignals (and colonialism) rock always beats scissors.

The only National Monument ever to go missing for nearly 50 years then resurface in a dump.

Before Nixon, "watergate" meant canals.