This astonishingly geometric cave has inspired everyone from Jules Verne to Pink Floyd.

This house was home to a succession of park keepers for more than 90 years, but in all that time, it never had a toilet.

3,000 family pets and military animals are buried in this London graveyard.

The pioneer of psychoanalysis spent the final year of his life here, still seeing patients on that famous couch.

The world's oldest optical museum is an odd collection where the exhibits look at you.

Buried for centuries, this hidden cellar is all that remains of London's medieval Whitefriars Monastery.

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

These mysterious Viking chess pieces spent centuries hidden on a remote Scottish island.

Hidden in the courtyard of the Victoria and Albert Museum is a little-known tribute to two celebrity pooches.

It's a mystery why these remains were found with no traces of their bodies in sight.

This Viking gravestone carved in a rare runic style was dug up from a London churchyard.

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

These morbid carvings remind churchgoers of the inevitability of death.

The medieval woodcut-style artwork chronicles life in medieval London.

A plaque pays tribute to the artist's longtime London home.

This open-air mosaic gallery honoring an English artist took hundreds of volunteers more than seven years to create.

Nestled in the mountains of Portugal is a tiny village made almost entirely of schist.

An abandoned anti-smuggling station decays along the boundary between Portugal and Spain.

This little-known national museum houses over 40,000 works of art from across the world.

The extravagance of Portugal’s "Pink Palace" resulted in a noble title being awarded to its owner.

When this former hospital pharmacy closed in 2013 it already looked like a museum, so they turned it into one.

This abandoned fortress still stands guard over the Guadiana River despite a history of warfare.

The walls of this small village have been turned into a living street art gallery.

An abandoned mining complex in Serra do Gerês.