These three pillboxes still remain as a witness of the Spanish Civil War in a park of Madrid.

This nondescript building hides an entrance to an underground nuclear tunnel system.

Come face to face with the ferocious visage of the serpent-headed mother goddess of the Aztecs.

Fragments of human bones are embedded in the walls of this quaint alley.

This abandoned viewing platform has become a monument to unnecessary government spending.

Legends surround this natural rock formation that looks like a Moorish king wearing a crown.

Stockholm's narrowest alley is barely over two feet wide.

This centuries-old rooster survived a devastating fire.

A wooden street sign is the last remnant of this 400-year-old alleyway.

This interactive museum highlights the history and philosophy of one of the world's most difficult sailing races.

A gruesome and mysterious ancient sculpture of a pagan deity from the Roman occupation of Celtic Britain.

Outside a Viking museum sits a modern-day runestone made by a Swedish runemaster using traditional methods.

This intriguing Bronze Age artifact spent centuries hidden within a Welsh faerie hill.

This museum houses two of Mexico's most impressive Aztec monoliths.

When this 18th-century observatory's meridian was moved, it set Sweden's time back by 0.04 seconds.

Once home to an aristocratic family and a workers' organization, this intricately tiled building now houses a chain restaurant.

This technological marvel is one of the world's first 3D-printed bridges.

A plaque marks the house in Stockholm where the French philosopher René Descartes lived up until his death.

The striking jade death mask of an ancient Maya king is displayed in a replica tomb in Mexico City.

This statue and park honor the Swedish chemist Jacob Berzelius, one of the founders of modern chemistry.

The grand complex of buildings that comprise Liverpool's Exchange Flags includes a vast World War II bunker that has been frozen in time.

This enigmatic artifact is a rare depiction of an ancient Iberian noblewoman.

The ruins of the bathhouse used by the ill-fated last Aztec emperor still lie in Chapultepec Park.

This ancient jade mask depicting the Zapotec bat god was found in the ruins of the pyramids of Monte Alban.