Europe's longest Hornbeam tunnel.

This cannonball has been lodged in the church wall since a 1695 bombardment destroyed the city center.

The private train station King Leopold II built for the Belgian royal family now lies abandoned.

This interactive sculpture allows visitors to leave a message in case a friend passes by.

You can spot this teeny tiny stained-glass window from the Bonifacius bridge.

Each time a baby is born in Ghent, these streetlights flash in silent celebration.

This narrow Ghent alleyway is an officially sanctioned street art tunnel.

A fantastical 13th-century fortification sandwiched by a modern hotel in central Brussels.

This recently restored crypt was once plagued by liquefying coffins and exploding caskets.

This fairytale castle was built as a show of power but was nearly torn down thanks to its history of torture.

Each spring, a dreamlike carpet of bluebells overtakes the forest floor.

A small hippo-shaped fountain in Berlin, Germany.

Dine at a legendary after-hours joint frequented by David Bowie and Iggy Pop.

Berlin's old airport.

The site of the famous Nazi book burning campaign of 1933 remembers its "Night of Shame".

A whimsical and twisted animatronic monster tour beneath the streets of Berlin.

Metro station with a claim to rock n' roll fame.

A monument to the three wars that led to German unification towers within a Berlin traffic circle.

Reminders of the Battle of Berlin still scar the railway.

These former Nazi anti-aircraft towers offer a singular view of the north of Berlin in the summer, and a sanctuary for bats in winter.

According to the Guanches, the aboriginal people of the Canary Islands, the king of evil lives inside this volcano.

This huge limestone slab is considered the largest paving stone in Belgium and a relic of the earliest modern paving in Brussels.

A shining memorial to a Belgian hero is said to bring luck to anyone who touches it.

Head underground and take a tour of the city's working sewer system, and see the underground river Senne, which defined Brussels for centuries before being covered in the 19th century.