Crossing the Unthinkable: The World's Most Dangerous Bridges, photo 13

The Inca Bridge at Machu Picchu in Peru is not for the faint-hearted. Carved into the cliffs of the Andes, this narrow stone bridge was once used by the Incas to cross deep ravines. The bridge itself is very narrow, with no barriers to stop a fall.

Today, tourists can still walk across it, though many prefer to admire it from a distance, as the drop on either side is dizzying.