Legend says this sacred tree was a gift from goddess Athena.

The former Nazi Germany concentration camp stands as a museum to remember this dark chapter in European history.

St. Mary's features one of the most spectacularly decorated cathedral interiors in all of Europe.

Historic fortifications form the boundary of Vatican City, but it’s not entirely surrounded by walls.

Hidden beneath the famous St. Peter's Basilica is a veritable city of the dead that may hold the remains of St. Peter himself.

Once featuring over 60 miniature replicas of famous Chinese locations, the Splendid China theme park was completely abandoned and is now mostly rubble.

This abandoned NASA launch site is now a memorial to a trio of astronauts who were killed there.

A building so large that it has its own weather.

Quite simply the largest tree in the world (by volume).

After starting as a whimsical tourist attraction that eventually found its way to ruin, these renovated canals offer a lush and exotic locale on the Pacific Coast.

These markers are used to document elevation changes around the famed dam.

The arch that celebrated the completion of the Lincoln and Victory highways.

This Las Vegas casino boasts one of only three spiral escalators in the United States—an engineering wonder.

This beloved roadside attraction welcomes visitors with his signature neon smile.

These markers are used to determine the ground stability of the Strip.

Experience a charming 1930s version of "infotainment."

Giant, natural amphitheaters made of delicate geological formations called "hoodoos."

World's largest barrel containing an inn.

These medieval skyscrapers dominate the skyline of this small Tuscan town.

Featuring the last fully walkable parapet walkway in Europe.

This bustling Italian plaza is built on the ruins of an ancient Roman amphitheater.

Fortified medieval tower house with lovely roof garden.

Next to the Leaning Tower of Pisa is a cemetery home to ancient sarcophagi and disturbing murals.

The only visible reminders of an elaborate engineering project that kept the tower from reaching the tipping point.