An ancient Viking runestone hidden in plain view in Stockholm's old town.

The remains of Alfred Nobel's first nitroglycerine factory can still be found on the shores of a Swedish bay.

This homeless brass fox tugs at your heartstrings from the foot of a Stockholm bridge.

Now a hotel, this former bank was the site of a robbery that spawned a psychological phenomenon.

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

The world's largest model of our planetary system.

The library's graceful rotunda is like a bibliophile's temple.

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

It houses the remains of a 17th-century version of the Titanic.

Sweden's first nuclear reactor, R1, can be visited 25 meters underground.

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

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

London's only public statue of the much-married monarch adorns the gate to St Bartholomew’s Hospital.

One of the greatest movie directors of all time lived and worked in this London flat from 1926 to 1939.

Tucked away in the Tower of London is the weapon once used to execute high-profile prisoners.

Where condemned pirates met their grisly end at the gallows on the River Thames.

A labyrinth hides in plain sight at each of the city's 270 Tube stations.

Tombstones and enigmatic inscriptions hide within a vault beneath a popular London arts venue.

This long-hidden space above Westminster Abbey now displays the battle gear of the legendary King Henry V.

The 2,000-year-old beam is tucked within the pedestrian entrance to the old London Bridge.

Never actually a tree at all, this spot was the site of London's public hangings for nearly 600 years.

These official units of measurement—including chains and perches—lie hidden beneath tourists' feet in a popular London hub.

This pump on the bad side of Victorian London betrayed a horrifying revelation about the water supply.

Three small rooms at the back of the “official” church of London's clowns.