A lush indoor jungle fills this bustling Madrid transportation hub.

Abandoned for decades, trains still pass through the now-restored remains of one of Madrid's first metro stations.

Hundreds of hidden passageways weave through the old quarters of Lyon.

This hidden garden boasts amazing views of Lyon.

Got apples?

Where the high court feasted, frolicked, and high courtesans put their royal lovers' diamonds to the test.

The oldest stone house in Paris was built by its most famous alchemist.

The vast, legendary catacombs hold secrets much stranger than stacked bones.

This hydraulic power system in London was, at one time, seen as a rival to electricity and the last of its kind to shut down.

These docile dragons are ideal inhabitants for the conservatory greenhouses.

It is still illegal to "quarrel" at this unlikely urban oasis built on top of an electrical substation.

A decommissioned tube carriage within a London museum hosts a regular pop-up supper club.

A network of concealed passageways lies within a busy London Underground station.

English antique dealers with a taste for the unusual.

An alleyway plaque is all that remains of the independent Texas' envoy that forgot to pay their rent for over a century.

The world's first underground farm is in a World War II bomb shelter beneath the streets of London.

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

Hidden beneath the streets of London is a secret subway that only carried mail.

The perfectly preserved underground rooms where Churchill plotted the war against Germany.

A 300-year-old tea shop that brought tea to the English people, not to mention the Queen herself.

Vast network of air raid tunnels beneath Lombardy's capital.

The secret medieval communication system will whisk your words above the noise of the crowd.

Long-abandoned art nouveau baths beneath Milan's Piazza Oberdan.

Two very green residential towers cleaning the air in one of Europe's most polluted cities.