Mother Nature Network: Australia's Great Barrier Reef, one of the seven natural wonders of the world, is already threatened by climate change, coral bleaching, pesticides, overfishing, oil spills and invasive species. Now we can add four unexploded bombs to the list. U.S. Navy pilots dropped the four bombs on the Great Barrier Reef this week during Talisman Saber, a joint exercise between the Australian and United States militaries. More than 28,000 soldiers and other military personnel converge on northern Australia...