An early Portuguese serial killer's head is alarmingly well preserved in a jar at the University of Lisbon.

A king's pet, a pope's gift, an artist's inspiration, and a tragic shipwreck.

An ancient and majestic fortress that played an integral role in Portugal's road to independence.

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

This unreal Portuguese palace looks as though it's made from a pile of different castles.

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

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

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

This once-derelict building is now one of the most beautiful bookstores in the world.

This upscale streetwear store is hidden behind a fake Snapple machine in the back of a deli.

Large, airy hallways hide within an impenetrable fortress that beckons urban explorers.

Monks turned this mountain into a temple by digging grottoes, tunnels, and a huge pagoda-like hall.

A more than 260-year-old crypt built on Boston's oldest English burial ground.

The only one of New Hampshire's original 14 World War II watchtowers to survive in near-original condition.

The hidden lair of an alchemist rediscovered in a 2002 flood.

Bullet holes in the church wall are a reminder of the violence that followed the assassination of a high-ranking Nazi officer.

The often-overlooked ruins of one of the first buildings designed by famous landscape architect Frederick Olmsted.

A beautiful Victorian-era cemetery, complete with a miniature village.

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

A unique columnar residence built in the 19th century.

The homestead of American patriot John Adams.

Black cat statues guard this temple-like Egyptian revival factory.

A quirky bookstore hidden beneath a sea of whitewashed Greek villas.

The remnants of a Gilded Age elegance preserved in the middle of a stark modern tower.