This colorful gate stands as an entrance to the walled city of Old San Juan.

The world's brightest bioluminescent bay.

The world's second largest single-dish radio telescope, once used to contact E.T.

Historic slum of Tartu.

Walk along the medieval fortifications that have been protecting Tallinn since the 14th century.

First point in the Struve Geodetic Arc, a 19th century undertaking to measure the Earth's size and shape.

For the last century, this international border within a small town has appeared and disappeared various times.

The oldest continuously operating pharmacy in Europe.

An exclusive waiting room built for the royal family in the 1920s is hidden in Milan’s central station.

The smaller of the three towers of San Marino is a former prison inaccessible from the ground level.

High in the clouds of San Marino sits an ancient tower with some curious prisoner graffiti.

A tribute to the "toughest woman on the Texas frontier," who survived a famous battle and five husbands.

This daunting 1st century fortress was purportedly the site of one of the most dramatic revolts of the Jewish-Roman wars.

This tree was extinct for a thousand years before sprouting again from a 2,000-year-old seed.

Descend 2,030 years into the past in an ancient tunnel built to support Jerusalem's most famous wall.

Once at the center of U.S. finance, the historic Trading Room was meticulously recreated piece by piece and rebuilt in the Art Institute.

A once-booming Mississippi River town with a history of racial violence is now eerie and mostly abandoned.

This modest post office sits on the site of H.H. Holmes' famous murder castle.

A 56-foot tall statue of the god of fire looks out over Birmingham, Alabama.

This lovely public park has an intriguing past and unique wooden bridges made of concrete.

The remains of this ancient fortress atop a manmade hill look as though they were built into a volcano.

This southwest-styled home in Santa Fe claims to be the oldest house in America thanks to an ancient bit of foundation.

What is it about mushrooms that makes people wish they lived inside of them?

Not to be confused with the iconic New York City skyscraper, this Chicago building is a vibrant hub for artists.