A 90-year-old underground water reservoir offers a more industrial kind of tranquility.

Barcelona's historic 19th-century sewers, whose foundations were laid in medieval times, are open to the public for exploring.

The true depths of these stunning Moroccan caves are unknown.

Wild lights and sound effects turn a walk through some caves into an eye-popping 1960s tourist attraction.

This picturesque Umbrian city has a subterranean side that has long remained hidden.

Hidden on the urban banks of a Manchester river are a series of abandoned waterside industrial sites.

A Victorian Gothic masterpiece housing a world-class collection of rare books and manuscripts.

A treasure trove of artifacts, books, and artwork from the "golden age" of exploration.

New York's oldest and possibly grandest keeper of books.

This little-known collection holds everything from "Dracula" notes, to the manuscript of "Ulysses," to the forged works of Shakespeare.

Two thousand artifacts from around the world collected by one woman who loved to travel.

This Dutch-styled residential building was originally a cocoa company's World's Fair showpiece.

This Boston medical museum features the skull of the famous medical case of Phineas Gage.

America's most famous museum of medical oddities is home to the remains of Albert Einstein's brain.

An eccentric collection of antique items shares space with a prop-quality witchcraft museum.

This museum tells the story of lake dwellers from 4000 BC to 850 BC, replete with stilt houses and goats.

Enter the cabin of a life-sized Hindenburg replica.

These spooky and unusual stone formations create an underworldly landscape.

A communal oasis on Lake Zurich.

Kitschy horror displays now fill the house where some very real serial killing took place.

An alchemist's "magic door" stands in the middle of a Roman park.

"In the Dark" Restaurant in Paris offers dinner in complete darkness.

The oldest stone house in Paris was built by its most famous alchemist.

A devastated ruin in the south of Belize where British explorers claimed to unearth the Mitchell-Hedges Crystal Skull.