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

Collapsing castle on the Hudson river was once the storage site of military surplus business.

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

An enormous park-like oasis, full of famous Chicagoans' graves.

A reproduction of earth's many biomes.

Rare recess caves fill this unbelievably beautiful Ohio park.

A religious-themed park filled with folk art to bring biblical stories to life that fell into disrepair and now being restored.

Oregon's unexpected high desert, displaying layers of time in a colorful show.

Rustic natural hot springs in the deciduous rain forest.

A clear blue swimming hole with hidden caves, still unexplored.

A room in New York City that contains 250 cubic yards of dirt worth a million dollars.

See massive fossils that are over 200 million years old in northeastern Arizona.

A meditative space out of a fantasy story.

Giant, natural amphitheaters made of delicate geological formations called "hoodoos."

Quite simply the largest tree in the world (by volume).

This maze-like slot canyon is a unique gem within an already fascinating geological area.

Mural made of 100,000 pieces of hand blown glass, and until recently, the largest glass mural in the US.

The mysterious wonder of an underground waterfall is illuminated by multicolored lights.

Hundreds of glow-in-the-dark objects light up a museum in a historic zinc mine.

The scuba diving training facility was named for the steam that rises from the cave's chimney.

Couple's personal collection of pinball machines turned public pinball museum.

The ghostly remains of an accidental sea.

A whimsical village of unique treehouses resides in a Washington forest.

The largest hot spring in the United States is, as the name suggests, a stunning show of natural color.