The walls of the iconic dome are hollow and have a secret stairway.

A museum cafe showcases Native American dishes and indigenous ingredients from across the Western Hemisphere.

A historic estate packed with George Washington's heirlooms, and its own nuclear bunker.

The site of the climactic scene from the classic horror film is now a historic landmark.

St. Augustine's record-setting narrow street was designed to protect against pirates.

The walls of America's oldest masonry fort famously "swallowed" cannonballs.

An 18th-century Spanish watchtower made entirely of seashell and concrete.

It's a pretty creepy lock-up.

America's first public mental health facility.

With over 100 jam-packed rooms to explore plus elaborate tea services and events, the Mansion on O Street is a hidden treasure.

The United States Capitol's former columns still stand.

There's a crumbling old mansion inside the Smithsonian National Zoo.

Home to items never before seen by the public.

An eerie smiling woman looms above the granite tombstones.

Oldest unrestored plantation house in America that is open to the public.

A circle of stones on a hilltop in Peru’s high jungle, aligned with the winter solstice and largely ignored by archaeologists.

These overlooked ruins give an intimate look at the Inca Empire's urban planning prowess.

These ruined Incan mausoleum towers still bear carvings of sacred animals on their blocks.

An Inca spa, military outpost or maybe… both?

Mystic Incan rock piles three miles above sea level.

The sophisticated hydraulic system of the Nazca civilization.

Ancient Peruvian fertility temple or modern money-making hoax? Either way these monoliths still look like penises.

This geometric feat is a testament to the Incas' mesmerizing architectural abilities.

A kitschy Italian medieval-style castle incongruously located amidst one of the most historic towns in the Peruvian Amazon.