A hidden network of subterranean passageways lies tangled beneath Indianapolis' bustling City Market.

The only swimming pool listed on the National Register of Historic Places is emptied and refilled each day with naturally filtered water.

A hidden beach at the end of a tunnel of mangrove trees.

This ancient sinkhole-turned-healing-pool is a treasure trove of archeological discoveries and may even be the Fountain of Youth.

Possibly the most beautiful carillon in the world, set atop Iron Mountain in Florida.

Manatees congregate around a power plant's discharge canal where the water is especially warm.

The culprit who accidentally lit this 3,500-year-old tree on fire stated "I can't believe I burned down a tree older than Jesus."

Meet manatees face-to-face at this zoo for native species.

Where manatees are the snowbird vacationers in these balmy Florida springs.

Bad farming practices caused this unnatural natural wonder.

The ruins of a cotton mill that was burned in the Civil War are still hidden in a Georgia forest.

World's largest college campus includes a wildlife sanctuary and a former movie set on more than 28,000 acres.

In the Shawnee National Forest, amazing rock formations peer through the treeline.

A once-booming Mississippi River town with a history of racial violence is now eerie and mostly abandoned.

This park contains one of the largest concentrations of cedar glades, a unique and rare ecosystem.

Railroad came. People came. Railroad left. Town died.

All that's left of the quintessential "cabin in the woods."

Wild lights and sound effects turn a walk through some caves into an eye-popping 1960s tourist attraction.

An abandoned town is hidden in Great Smoky Mountains National Park.

Mile-long cave system near Great Smoky Mountains.

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

More than five miles of underground passageways.

This limestone outcropping was once a ceremonial site for ancient Native Americans.

An outdoor urban gallery of large-scale artworks by the world's most influential street artists.