Scattered throughout London are ancient remnants of the city's former bounding wall.

London's smallest police station is barely the size of a phone booth.

This 1920s Dutch barge is now a floating bookstore.

A 1,215-foot tunnel transports pedestrians beneath the River Thames.

An unusual round church in London with a Templar past.

An small and easily overlooked archaeology museum chock full of Egyptian artifacts.

The Tudor-style cottage in the middle of the square is not quite what it seems.

A measuring mishap led to these artificial stone ladies losing their stomachs.

The sixth of London's "Magnificent Seven" cemeteries, and a source of unlikely inspiration to Beatrix Potter.

Rebuilt remains of a temple to Roman god Mithras.

This ornate Victorian marketplace was the setting for Diagon Alley and the Leaky Cauldron in the Harry Potter films.

An easily overlooked stretch of ornate window fronts and gaslight lamps that could be right from the pages of Dickens.

This impressive rupestrian engraving dates back to the Upper Palaeolithic period.

In Portugal, a secluded stretch of untouched natural beauty hides at the bottom of a steep hill.

A unique seaside cave where Aleister Crowley faked his own death.

This eccentrically decorated palace is replete with grottoes, fountains, tunnels, and caves.

Famed for its pie, this eatery has a hidden tunnel that allowed Catholics to secretly walk to church during the Reformation.

The last of Amsterdam's original sea level markers remaining in place.

Tucked in the back of a flower shop, its collection details the fascinating history of the Netherlands's iconic symbol.

This museum housed in a former brothel explores the past and present of sex work in Amsterdam.

This cheese shop also houses a museum devoted to Dutch dairy.

This bird-loving brewery is situated in an old municipal bathhouse, nestled beside a 300-year-old windmill.

One of Amsterdam's narrowest houses is located right across the canal from its widest.

Christ's blood preserved in a Belgian town.