Once the smallest park in the national system.

Generations of rockhounds got their start at this beloved, historic mine.

At the most northwestern point of the contiguous United States lies a trail leading through an ancient forest to stunning cliff-side vistas and massive oceanside caves.

The maximum security facility was once thought to have "the bloodiest 47 acres in America."

Over thousands of years, this slot canyon was carved out of the bed of an ancient lake.

An urban forest overtakes what was the greatest failure of utopic urban housing.

A subterranean haunt where wine and homemade biscottis are always on the menu.

These pink granite water slides are the result of millions of years of erosion.

The world's largest man-made underground caverns offer unparalleled underwater wonders.

In the St. Francois Mountains in the Ozarks at 1,772 feet above sea level, Taum Sauk Mountain is the highest (and possibly oldest) peak in Missouri.

Around 50 million gallons of water flow from this spring every day.

Once an ambitious nature spa, these forest ruins now provide a serene monument to failure.

The largest riverfront opening cave in the Ozarks.

An underground river pushes more than 200 million gallons out of this Missouri spring every day.

Often referred to as the largest spring in America.

Erosion and time have created a naturally-formed water park.

An abandoned factory along the Mississippi coated by a giant mural.

During the 19th-century, this park was a vital trading fort.

An abandoned meat mecca now serves as an excellent locale for urban exploration.

A late sculptor's last project was to turn this abandoned cement factory into a castle-themed amusement park.

First known as the "Sample House," this construction mash-up was meant to be a living catalogue of building styles.

After miraculously surviving a giant flood, the 19th-century nut tree and its offspring are protected with their own tiny park.