A series of concentric circles represented a bold new experiment in socialist urban planning, and it notoriously difficult to get around.

A spectacular Petrine Baroque summer palace commissioned for Catherine I by Peter the Great.

This set of concrete chambers resonates according to different notes of octaves.

The oldest church in the Estonian capital is home to over 100 fantastical cemetery coats-of-arms.

The oldest continuously operating pharmacy in Europe.

Only a fragment of this 15th-century Dance of Death survives today.

This medieval Estonian tower provides access to a series of tunnels that have been home to soldiers, punks, and fine art.

"There's nothing there," and other lies the KGB told us.

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

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

An atomic bomb-proof strongbox protects the U.S. Constitution from terrorists and thieves.

A historic neighborhood church is reborn as a psychedelic arts collective.

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

Fittingly, America's museum of architecture is itself a magnificently designed old building.

Longest fortress system anywhere in Europe.

When a supermarket opened up shop within a medieval building it became protected by UNESCO on a technicality.

An illuminated manuscript comprising the life's work of a lone monk, inexplicably decorated with a portrait of Satan himself.

Dine on food tied to Viking history and legend at this subterranean Stockholm spot.

A hidden bathhouse in one of Stockholm's oldest and busiest neighborhoods is open to anyone who can find it.

Stockholm's decidedly unique public transit system has doubled as the world's longest art gallery for decades.

Stockholm's deepest subway station is also a stunning ecological wonder.

From toys to kettles to scientific instruments, this German museum celebrates the beauty of 20th century design.

The most visited, and possibly most controversial, museum in Germany.

A surreal museum of extraordinary industrial objects.