TO GO WITH AFP STORY Science-space-impact-Tunguska,FEATURE by Richard Ingham(FILES) This handout file image obtained on March 9, 2005 from NASA shows the Meteor Crater in Arizona. A hundred years ago this week, on June 30, 1908, a gigantic explosion ripped open the dawn sky above the swampy taiga forest of western Siberia, leaving a scientific riddle that endures to this day. What caused the so-called Tunguska Event in Siberia, named after the Podkamennaya Tunguska river near where it happened, has spawned at least a half a dozen theories, including the idea that a meteor similar to what happened in Arizona some 49,000 years ago. RESTRICTED TO EDITORIAL USE GETTY OUT AFP PHOTO/HO/NASA