This ancient Roman prison is decorated with an upside-down cross not as blasphemy but as saintly tribute.

St. Francesca Romana, patron saint of drivers, resides in the church she founded.

Jesus' footprints and the first catacombs in the world.

At the crypt of St. Mary of Eulogies and the Dead you are left alone to ponder mortality among piles of skulls.

The underground sanctuary of a mysterious ancient cult.

The engineering wonder and pleasure dome of a much-maligned emperor still holds undiscovered secrets.

The crypts of Capuchin friars decorated with the bones of over 4,000 friars, including an entire "crypt of pelvises."

This picturesque abandoned home is rumored to be a spite house.

Once defending the city of New Orleans from invasion by sea, this fort faces an uncertain future.

Curious rock formations in a tiny Kansas state park.

Aqueducts have dramatically changed this old lake, now home to tufa towers and its very own species of tiny brine shrimp.

A sacred site for Native Americans surveyed by George Washington and owned by both King George III and Thomas Jefferson.

Only intact Ichthyosaur skeleton in the United States.

Labor provided by Great Depression-era programs restored these 25,000 acres in Tennessee.

A 300-yard long natural tunnel runs through a ridge deep in the Appalachian Mountains.

This picturesque cliffside beacon no longer calls to sailors but shines once a year in honor of a famous shipwreck.

One of the most scenic falls in Massachusetts has a history of death and grim legend.

This narrow canyon has been carved by the river over thousands of years into spectacular and curious forms.

This Florida sinkhole hides a rainforest-like micro-environment in its depths.

Even Henry David Thoreau found this glacial pothole irresistible.

A deteriorating 1990s Girl Scout camp in a beautiful lush park on Lake Ontario.

An abandoned network of military bunkers and tunnels open for anyone willing to descend into the darkness.

The ruins of an abandoned 18th-century town that is slowly being reclaimed by the forest.

Lines from a walking song are etched into a rock in Natural Bridge State Park.