John Baez's excellent Azimuth project has a really good summary of current scientific understanding of mass extinction events: http://www.azimuthproject.org/azi

John Baez's excellent Azimuth project has a really good summary of current scientific understanding of mass extinction events: http://www.azimuthproject.org/azimuth/show/Extinction http://www.azimuthproject.org/azimuth/show/Extinction.