The charred, dead skeleton of an oak tree is said to have consoled a dying 16th-century Italian poet.

Rome's first modern graffito is all that is left of a peaceful protest from the 1970s.

This small Roman arch was transformed into a dwelling by the founder of the Trinitarian Order.

This one-of-a-kind Roman arch is not actually dedicated to the god Janus.

Hundreds of lucky felines haunt the ruins where Caesar was murdered.

The ruins of the largest gladiatorial school in Rome are largely hidden under modern streets.

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

This hidden Art Nouveau courtyard is an opulent feast for the eyes.

This ancient market is often referred to as the oldest shopping mall in the world.

A forgotten "city of water" underneath the famous Trevi district.

An alchemist's "magic door" stands in the middle of a Roman park.

Architectural monsters are devouring this palace's every door and window.

This Roman arch marks the location of an ancient gate at one of the Seven Hills of Rome.

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

This statue is dedicated to the animal cosmonauts that perished during space flight experiments.

The former home of the man whose radical ideas changed the nation.

Grand architectural relics gain a new life in this Pennsylvania warehouse.

Hidden in the forest atop a long, winding driveway is this stately home.

Built to defend Washington D.C. during the Civil War, this fort became a post-war nucleus for a thriving Black community.

This stately building in Alexandria, Virginia was built by Freemasons to honor one of their most famous members.

This brick-lined pedestrian walkway was once a railway tunnel used during the Civil War.

This once-palatial plantation may have been the site where a draft of the Declaration of Independence was written.

America's first public mental health facility.

Giant busts of 42 U.S. presidents are sitting in a field in Virginia.