Callaghan/Tillman Memorial Bridge, Arizona: This bridge that sits high over the Hoover Dam is as much a wonder as the dam itself. We were told it was a must walk across. It's so high up that when we drove across it two days earlier we didn't even know we were over the dam. Damn! So we drove up to the adjacent parking area, hiked up the stairs and walked across it with many other sight seekers. I'll admit I'm not good with man made heights but this was absolutely spectacular.