This abandoned subway station beneath the Waldorf-Astoria hotel is allegedly used to secretly transport presidents.

This beautiful building in the heart of Manhattan's Financial District was empty for decades.

A crumbling hospital from the 1850s on Roosevelt Island.

A tiny museum housed in a New York freight elevator specializes in the "overlooked, dismissed, or ignored."

Venice's "Island of the Mad."

This Venice bookstore has resigned itself to constant flooding by keeping its books in bathtubs and boats.

An enchanting monastery in the Spanish countryside appears lifted from a fantasy tale.

A science fiction-inspired apartment building designed by a famous Spanish architect.

A once grand hotel now sits eerily abandoned in the Catalan countryside outside Barcelona.

These secluded and abandoned anti-aircraft defenses have become one the most scenic viewpoints in Barcelona.

Construction of Barcelona's iconic (but controversial) church is expected to be completed in 2026—a century after the death of its architect.

This hidden Art Nouveau courtyard is an opulent feast for the eyes.

Ancient ruins hidden underneath a modern luxury hotel.

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

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

This innocuous-seeming building is actually the remnant of a major wartime brothel.

Little island known for its massive beachfront "flowerpot" pillars.

This natural sea cave hides a small wonderland of natural turquoise beauty.

A long-abandoned provincial prison left to crumble in the frigid air of Ontario.