The Meteor Crater just outside Flagstaff is incredible!  This thing is 550 feet deep, almost a mile in diameter, and 2 1/2 miles in circumference. One of their analogies was to picture 20 NFL games being played simultaneously on the floor of the crater, while 2 million fans watched on the embankment and edge.  Also the Washington Monument would be short of eclipsing the crater edge, from the floor.  I'm glad we stopped to see it.