Australian Lake set aglow by Noctiluca Scintillans.

An abandoned railroad tunnel is now filled with bioluminescent bugs.

An Australian lake whose pink hue defies scientific explanation.

Street artists transformed this abandoned industrial neighborhood into one of Europe's largest graffiti exhibitions.

This knockout museum lies in a sleepy corner of London.

A wonderfully eclectic Victorian aquarium lies in the basement of the Horniman Museum.

These morbid carvings remind churchgoers of the inevitability of death.

Traveling collection of amazing automata.

Wedged between two buildings at the Great Ormond Street Hospital is this secret music installation.

A special shop catering to the needs of things that go bump in the night, and also helps kids become writers.

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

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

A breathtaking flower tunnel bursts with beautiful golden blossoms.

A subterranean playground in an abandoned slate mine.

In the days before house numbers, these colorful signs held clues about various buildings' purposes.

The Devil’s own choice when he needs a drink in Brussels.

An "inside out" building in the historical center of Paris, featuring Europe’s largest modern art museum.

Seven mini rowhouses fill the gap between 54 and 70 Westerstraat.

This city square is home to Amsterdam's oldest church and a statue honoring Dutch sex workers.

A mysterious pair of mice eating a piece of cheese is London's smallest public sculpture.

An incarceration museum on the spot of possibly the oldest prison in England.

A Portuguese Jewish cemetery filled with stunning grave monuments that date back to 1614.

Three connected pink triangles memorialize persecuted gays and lesbians throughout history.

This unattributed sculpture in Amsterdam is hiding in plain sight.