December 21, 2012 - the purported last day of a 5,125-year cycle in the Mesoamerican calendar has been added to an endless list of days when the world has been expected to end. But what are our real chances of being wiped out by a catastrophic event - the kind that has happened in the past and will inevitably occur again someday? In the August issue of EARTH, we explore four of the most probable global events that could change life on Earth forever.