A 140-year-old castle with underground caves hidden in San Francisco.

During prohibition, corrupt city officials ran drinking dens under the streets of Downtown Los Angeles.

This iconic L.A. bookshop is housed in an abandoned bank—both symbolic and chic.

Bring your own cardboard to the slippery slopes of concrete hidden in a neighborhood park.

World's largest collection of serial killer artwork and other macabre exhibits.

A curious collection of locks and keys.

A subterranean shelter remains intact under a busy Paris railway station.

An unusual number plate signifies the building's saucy past.

The spiral staircase hidden inside this giant Parisian column was likely built for Catherine de Medici's astrologer.

This Parisian café sticks almost exclusively to an ethos of reuse.

135 bronze medallions mark a French meridian line which once rivaled the hotshot Greenwich timeline.

An all-American grocery store in Paris sells U.S. "cuisine" like Pop-Tarts and Heinz ketchup to homesick ex-pats.

The first green space constructed on an elevated viaduct.

Remains of Roman amphitheater hidden in a sleepy quarter of Paris.

An overlooked marble is the only surviving in-place 18th century example of the measurement that changed how we define distance.

A lone cannonball remains lodged in the walls of the Hotel de Sens as a sober reminder of a revolution.

This Parisian fountain is a little bit fantasy and a little bit nightclub.

A majestic lion statue guards this Parisian botanical garden, and feeds on an unsettling item.

The curious underground history of keeping Paris clean.

Egyptian Revival movie palace abandoned for decades and restored in 2013.

A grotto-like lodge decorated with gilded mosaics, rocks and a noisy waterfall was the setting for the most sumptuous pleasure house of 1900s Europe.

The history of radiation is on safe display at the foot of a radiation science laboratory.

Indents in the pavement are all that remain from a grisly death machine in Paris.

A scandalous Art Nouveau collaboration that set Paris all atwitter at the turn of the century.