Doomsday scenarios seem to be getting more common than solar eclipses. (Remember Harold Camping, the radio minister who declared May 21, 2011, as the day the world would end -- and then revised his doomsdate to Oct. 21?) The latest prophecy comes from the ancient Mayans, who supposedly identified Dec. 21, 2012, as the day of doom. NPR's David Greene talks with David Stuart, an expert on the Mayans from the University of Texas.