A forgotten "city of water" underneath the famous Trevi district.

This park on the outskirts of Rome boasts two ancient Roman aqueducts and a medieval tower.

An Italian ghost town that fought obscurity, until a mysterious illness brought it to its knees.

An abandoned Roman salami factory has found new life as an art space home to 200 squatters.

A mysterious chasm in the heart of the ancient Roman Forum, the Lacus Curtius was once believed to be a gateway to hell.

A scale model of the world's largest existing lensed telescope.

This railroad track was so secretive that it was omitted from all maps.

Stockholm's last remaining paternoster elevator.

Eat a Nobel banquet from any year in the basement of Stockholm's City Hall.

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

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

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

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

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

A stunning cemetery famous for its extraordinarily delicate & lifelike mourning sculptures.

This strange bovine gargoyle is said to have been put in as a passive aggressive form of revenge.

This museum is rotten with a century of wax medical deformities.

One of the largest caves in the world open to the public.

Now abandoned, the world's first torpedo factory is crumbling into the sea.

Daily jousts and dancing barrel makers in Munich's mechanical clock.