A little-known subterranean street runs under some of the best-known buildings in Chicago's River North.

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

An annual festival celebrating Colorado's most famous frozen corpse.

Hidden beneath a bridge, this waterfall is known for the large amounts of graffiti painted on its base.

The ancient dwellings of the Puebloans, located in the cliffs of majestic mesas.

The largest self-built castle in the U.S., six decades in the making.

A tribute to the twin ideals of religion and patriotism, built by a Catholic priest from rocks, glass, and innumerable found items.

A fairytale art house in the middle of Wisconsin.

Literally a million fascinating cartographic artifacts are held in this sprawling university collection.

An off-the-beaten-path place to literally hang beneath Milwaukee's Holton Street Bridge.

Five acres of divinely inspired folk art.

An ever-evolving wall of graffiti art off East Liberty Street.

This shallow cave got its name from copious amount of human bones found inside.

Though its branches are painted bright colors by generations of locals, the tree is said to be a haunted portal to hell.

Unique Michigan homes look like something out of Tolkien's "The Lord of the Rings."

The writing studio of an American adventure author, built in the style of a medieval castle.

The city is full of whimsical portals to another world.

This sprawling bookstore spans four floors and has more than a million books in stock.

House built with over 60,000 glass bottles.