This Estonian watchtower looks like a big tree with two giant bird nests on it, complete with a "golden egg."

This old and disused water tower has received a new life as an art gallery with a piano staircase.

This set of concrete chambers resonates according to different notes of octaves.

This abandoned fighter jet launch center is now a haunting street racing course.

The ruins of an ancient mansion sits in front of and underneath a contemporary art gallery.

This gem is often wrongly overshadowed by the larger, newer Metropolis church in Athens.

St. Mary's Asylum was isolated and abandoned before being converted into a Bed and Breakfast.

Hundreds of prehistoric stones, arranged in perfect lines in a French field.

This bridge was constructed from the rubble of the infamous Bastille.

Historic medieval fortified city in southwest France.

This French ghost town from World War II has been left as a memorial to German atrocities during World War II.

The water-powered carriages are long gone, but for over a century this funicular railway has been the fastest route to the highest point in Paris.

This statue was given to France by the United States to commemorate the centennial of the French Revolution.

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

Paris' linguistic guide to the international language of, yes, love.

This 1,300 year old monastery built atop a single rock was once only accessible depending on the whims of the tide.

Abandoned railway line circling the city of Paris.

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

360° views of the city plus five floors of optical illusions and hands-on science.

Edinburgh's oldest statue hides within a parking lot behind the city's iconic cathedral.

The 20th-century chapel’s intricate woodwork includes amazing details, most notably angels playing bagpipes.

A sweet symbol marks the place where torturous executions were once carried out.

This city square dates back to the Scottish Enlightenment and was home to some of Scotland's brightest minds.

The world’s first museum dedicated to the history of childhood opened in Edinburgh in 1955.