A giant portrait of an ancient king looms over the river.

A Roman monument to their own civil engineering prowess.

This massive subterranean fissure is known as the "underground Grand Canyon."

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

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

19th century pendulum and a clock restored by a rogue group of guerilla artists.

Hidden away in a modern library are the remains of one of the 18th century's most celebrated wunderkammers.

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

Abandoned railway line circling the city of Paris.

A mechanized clock where a man fights off a dragon, crab, and rooster.

Notorious artist squat renovated into legal studios.

A Parisian exterminator's shop whose storefront displays stuffed rats hanging from traps.

One of the world's steepest streets seems to have houses sliding down it.

An island in the middle of a lake, in the middle of a volcano, in the middle of a bigger lake, in the middle of a bigger island, in an archipelago, in the Pacific Ocean.

River pirates, murderous bandits, and serial killers used this Illinois cave as their debauched hideout.

These Great Lakes sea caves accessible by boat in the summer turn into temples of ice reachable by foot in the winter.

A massive underwater sinkhole made famous by Jacques Cousteau.

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

Stalactites and stalagmites in a beautifully lit cave.

Inhospitable caverns just beneath this island paradise offer insights into alien worlds.

Natural stone arches form a walkable "cathedral" at low tide, then disappear entirely when the water returns.

The glowing blue Nymphaeum of Emperor Tiberius.

Mother Nature outdoes herself with this stunningly beautiful set of caves carved into marble.

A serpentine highway twists and turns over the treacherous Norwegian Sea.