The world's longest known cave system.

The massive, original Colgate Clock is mounted on a defunct factory building that was once a prison.

The oldest operating Mississippi River–style steamboat still navigates the Ohio River.

This former tuberculosis research hospital endeavored to cure the dreaded disease with fresh air and positive attitudes.

Colorful sedimentary rock layers provide opportunities both for hiking and photography.

Finally the worlds of UFO enthusiasts and beef jerky lovers have come together in this roadside shop.

This giant temperature attraction created some giant electric bills.

A Nessie-like monster supposedly lurks in this lake.

The unluckiest U-boat in the entire German fleet is now on display in a Chicago museum.

A lush green oasis in the middle of Chicago.

Fragments of architectural wonders from around the globe are embedded in the newspaper building's walls.

The palatial McDonald's that inspired Wesley Willis.

Hidden on the 7th floor of a historic building, this Cajun spot features wall-to-wall bottles of hot sauce.

Each of these gorgeous, intricately decorated rooms is painstakingly crafted to the scale of 1 inch: 1 foot.

This historic hotel invented the chocolate brownie.

A 400-foot-tall tall map of Chicago makes this once-ignored building stand out.

Four glass boxes hover over 1,000 feet in the air to give visitors an unparalleled view of Chicago.

The last masonry skyscraper ever built reveals a transitional moment in architectural history.

This historic house museum that celebrates one of the world's most influential social reformers is also one of the most haunted locations in Chicago.

One of the few remaining relics of the meatpacking hub that made Chicago the "hog butcher to the world."

These abandoned grain silos and tunnels are an urban exploration dream.

A chance to follow in the footsteps of the word's most legendary animator.

What was once one of the country's largest confectionary plants is now an abandoned ruin, but is still pretty sweet.

The clothes-washing king doubles as a community center with some unusual features, including an aviary filled with finches.