A stone arch leads to a cavernous 1930s spring house tucked away in the Pennsylvania woods.

Aqueducts have dramatically changed this old lake, now home to tufa towers and its very own species of tiny brine shrimp.

This Big Sur waterfall drops 80 feet directly into the Pacific Ocean, and inspired the hillside house up top.

An extraordinary mixture of natural beauty and man-made phenomena.

The last remaining relic of San Francisco's glittering 1915 Panama Pacific Exposition.

St Bernard de Clairvaux Episcopal Church, better known as one of the oldest European buildings in the Western Hemisphere.

Stripes of flowers bloom each spring in a 50 acre rainbow of color.

When ecosystems collide in the arid Southwest.

This lake in the Cascades lives up to its name, disappearing down a hole every spring.

Trails through lava-carved tube-caves display sparkling bacteria and a rich history.

The original toys that inspired the beloved children's stories are on display at the New York Public Library.

Sail through this subterranean show cave that is home to a legend that would go on to name nearly everything around it.

The oldest operating apothecary in the US has treated everyone from Thomas Edison to Mark Twain.

A tiny red pebble marks what may be the quietest outdoor spot in the United States.

A very classy 1920s-era bar right in Grand Central Station.

Canyon walls shrouded in ferns.

Enormous piles of historically significant stones, dumped by Congress in a forest, and abandoned for 60 years.

A 300-yard long natural tunnel runs through a ridge deep in the Appalachian Mountains.

The longest navigable underground river in the United States is home to countless blind and albino animals.

This road in the Great Smoky Mountains was supposed to assuage a displaced community, but ended up a $52 million dead end.

The oldest surviving bridge in New York City, which reopened to pedestrians in 2015.

A three-story copper skybridge connects two Manhattan buildings with Art Deco luxury.

The one-time watch house for incoming ships at the Golden Gate now stands abandoned and hidden in trees.

European castle ruins in an American state park are actually the product of death and grief.