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104 impact 250 million years ago at the boundary between the Permian and Triassic eras—the time of the greatest extinction of life known to science. Others believe that this extinction was triggered by terrestrial forces such as massive vulcanism. The two causes may not be mutually exclusive; massive impacts could trigger volcanic eruptions, perhaps at the antipodal point—the opposite side of the Earth.*° Two billion years ago, a massive body blasted out a crater 300 kilometers (nearly 200 miles) wide in what is now South Africa, the largest impact structure known on our planet.*' The effects on the Earth’s then-young biosphere would have been calamitous, dwarfing the event that killed off the dinosaurs. It tells us something that life survived. These collisions can have both negative and positive results. The Cretaceous-Triassic blast extinguished many species but it opened oppor- tunities for others, including those that evolved into humans. Catastrophism Revived Nearly all geologists once were in the grip of a nineteenth-century concept known as uniformitarianism, which claimed that geological change was gradual. The discovery of plate tectonics in the 1960s actually reinforced this doctrine, as the motion of Earth’s plates appeared to be very slow; geologist Walter Alvarez described them as moving at the speed of growing fingernails. Most geologists failed to relate impact craters on the Moon and Mars to the history of the Earth. As late as 1945, experts at the U.S. Geologi- cal Survey refused to acknowledge that Meteor Crater in Arizona was caused by an impact. Now we know that our planet has been pounded by large objects. Once geologists removed their mental blindfolds, they began finding impact craters all over the Earth. Such paradigm-changing discoveries can be serendipitous. The first evidence of the dinosaur-killing impact—a layer of rock unusually rich in iridium—was found by geologists looking for something entirely different. Impacts are not the only possible disasters. David Keys, in his book Catastrophe, contended that a massive volcanic eruption in 535 pro- foundly influenced our history. In a matter of decades, the old order died and a new world—essentially the world as we know it today—began to emerge.” We often are slow to react to potential extraterrestrial threats. The first Earth-crossing asteroid was discovered long ago, in 1932. Yet, worries about possible future impacts on the Earth did not motivate systematic surveys of such asteroids until the 1980s. Probabilities: Longevity