A marvel of Byzantine engineering under the streets of Istanbul.

In this historic region, homes and churches are carved into ancient volcanic rock formations.

An Italian crevasse filled with abandoned buildings gives visitors a look at the world without humans.

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

Architectureal fantasy quarter of Rome.

A 16th-century horror show built in a lovely Italian garden.

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

Hundreds of lucky felines haunt the ruins where Caesar was murdered.

The crypts of Capuchin friars decorated with the bones of over 4,000 friars, including an entire "crypt of pelvises."

Visit 17th-century ruins on a strikingly beautiful riverside walking trail.

Engineering ingenuity during the battle of Gibraltar.

Many of the tombs contain victims of yellow fever epidemics.

The defensive tower was built centuries ago to warn locals of Berber pirate attacks.

Red, white, and blue painted steps along with a Union Jack celebrate the outcome of Gibraltar's 1967 referendum.

Single runway interrupted by a busy road that cuts across its middle.

There was a top secret plan during World War II to seal a group of soldiers inside the rock.

Houses are tucked between, on, and underneath giant boulders.

A 16th century chapel decorated with bones, skulls, and entire bodies hanging from the wall.

The world's oldest bookshop still in operation.

Ring the bell to enter a bar that looks equal parts museum and old curiosity shop.

Legacy of the earthquake that nearly wiped Lisbon off the face of the Earth.

This eccentrically decorated palace is replete with grottoes, fountains, tunnels, and caves.

One of the most beautiful bookstores in the world hides a neo-Gothic interior behind an art nouveau facade.

Climb inside an iron crystal magnified 165 billion times its normal size.