This 19th century granite fortification is one of the most well preserved forts in New England.

More than five miles of underground passageways.

Visit this playground of waterfalls, caves and sinkholes.

Enormous lake at the bottom of a unique cave system.

Ghost stories surrounded this “castle,” the only structure from a huge mansion that was never built.

The Beaux-Arts aquarium boasts a vast collection of saltwater and freshwater fish on an island in the Detroit River.

The remains of a century-old mansion that met a fiery demise years after it was deserted.

This freshwater spring fills with 10,000 gallons of water per minute.

Island where motor vehicles are banned -- home to the only carless highway in the United States.

This national park on the shore of Lake Michigan features remarkable dunes that lie at the heart of a Chippewa legend.

Elevated freight railway turned wildly successful urban park.

Amid the bustle and noise of the Financial District hides a secluded garden oasis above the city streets.

A beautiful and abandoned New York subway station from 1904, complete with chandelier.

Long-abandoned trolley tunnels just a mile away from the White House are turning into an art space.

The United States Capitol's former columns still stand.

A failed piece of Orientalism has become the proud symbol of a community.

This eerily well-preserved coal town was saved from destruction by a Sean Connery movie.

Where America's first doomsday cult awaited the end of the world.

World's first "penitentiary," meant to be humane, drove men insane.

California's own medieval Italian castle, authentic down to the 300-year-old iron maiden in the dungeon.

These sub-farmland caves are home to a majestic stalactite known as the "Crystal King."

This fairytale-like structure is the last turf church ever constructed in Iceland.

The most powerful waterfall in all of Europe.

The unique waterfall in Iceland's south features a backdrop of geometric black lava columns.