These Great Lakes sea caves accessible by boat in the summer turn into temples of ice reachable by foot in the winter.

This cave holds a surprisingly colorful variety of geological oddities.

America's silhouette includes 100 square miles of wilderness jutting into Canada thanks to an old mapmaking error.

This boulder stays on its mountain thanks to a huge chain and on the map due to local legend.

Beautiful waterfalls grace a canyon visibly torn by a violent tornado.

One of the few surviving "Wigwam"-themed motels.

An enormous natural sandstone bridge in Kentucky.

An abandoned theme park with a strange and tumultuous past.

The world's longest known cave system.

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

Patrick Dougherty's fantastic living tree sculptures, mind-blowing but temporary.

A flamboyantly decorated bridge signals diversity and fellowship at the entry to Indianapolis' Broad Ripple Village.

This elaborate estate contains a stunning marble aviary, a sunken conservatory, and a magnificent music room with a two-story pipe organ.

Once adorning a New York skyscraper, these reclaimed ruins now haunt an Indiana park.

Symbolic attempt to overcome the issues of life.

A stunning, abandoned limestone quarry in southern Indiana provided the stone to erect the Empire State Building.

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

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."