Neither Spanish settlers nor anyone since has been able to tell who built these strange California rock walls.

Ancient human, animal, and geometric figures drawn in the earth.

Some of the tallest ancient American ruins continue to inspire awe and curiosity.

Many generations of travelers have passed through the steep, smooth walls of this canyon.

Perhaps the most spectacular grotto along the Algarve coastline.

A breathtaking 19th-century trail overlooks Lake Tahoe.

This large, circular tomb is the final resting place of the first emperor of Rome.

This once-popular otherworldly landscape was formed by one of the largest volcanic eruptions in history.

This sword isn't waiting for a king to pull it out. It was placed there by a saint.

One of the dwarven trees dates back to 1625 and survived the Hiroshima bombing.

A rare example of an ancient Japanese hill castle believed to be the fortress of a legendary demon.

Eulogizing an unknown man's unusual demise.

An Italian restaurant became a rock-and-roll hangout and the lair of the "Hollywood vampires."

Where a railroad loops under or over itself to rise or drop more than 70 feet in a short distance.

Castle and secret tunnel withstood a siege until its owner was killed by the Holy Roman Empire, while sitting on the toilet.

An immersive environment that is part haunted house, part choose-your-own-adventure, and part jungle gym.

One of the earliest depictions of an Arthurian scene that is shrouded in mystery.

On Japan's Tashirojima Island, cats are king.

Across Ring Mountain is a vast collection of Native American petroglyphs unique to the region.

An architect transformed this abandoned cement factory into his castle-like workshop.

Once the legendary capital of the Dacian Kingdom, this city has been reduced to mere ruins.

One of the best-preserved Roman amphitheaters is still used today, but for less brutal events.

The “Sistine Chapel” of the Middle Ages merges 11th-century science and religion in wondrous hues.

Inside the dome of this gigantic, Saturn-shaped building is one of the largest planetariums in Asia.