The subterranean "lake" below the Paris opera house inspired the Phantom of the Opera's lair.

Paris' fabled purveyor of exotic taxidermy and natural history curios.

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

Medieval museum built over Roman baths that features famous tapestries of a lady and a unicorn.

Exquisite wall sculptures carved in secret by an early quarryman in an obscure corner of the Paris Catacombs.

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

Behind the pink curtain, Au Bonheur du Jour, an art gallery of erotic archives.

France's national museum of scientific and industrial instruments.

This museum houses an imposing skeleton army comprised of creatures great and small.

High atop the Eiffel Tower is a small apartment built exclusively to entertain the science elite and make the rest of Paris jealous.

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

A haunting collection of the vanished and disappearing natural world.

One of the oldest medical collections in Europe focusing on rare surgical instruments.

A museum of carnival attractions where you are invited to ride the rides.

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

A horde of stray cats rule in this secluded Parisian cemetery under a bridge.

Not far from the busy city lie these remains of a time when the world was on fire.

This hole formed by a glacier is the largest and best-preserved of its kind in Stockholm.

An ancient tree that was once the favorite of a young Swedish royal.

A 130-year-old urinal is the oldest of Stockholm and perhaps all of Sweden.

The scars of a remarkably organized public protest to save 13 elm trees are still visible in this Stockholm park.

Outside a Viking museum sits a modern-day runestone made by a Swedish runemaster using traditional methods.

This grisly exhibit houses skeletons and weapons from a devastating 14th-century clash.

A golden egg-shaped sauna in the Arctic tundra.