American researchers have discovered new evidence that suggests a huge space rock crashed into Earth 250 million years ago. The discovery was published in Science magazine. Luanne Backer of the University of Washington led the study. She estimates the space rock, or meteor, was between 6 and l2 kilometers across. She also says the energy released by the meteor could be equal to an earthquake measuring 12 on the Richter scale. Researchers believe the force of the crash may have caused volcanoes to explode and spread liquid rock. Huge clouds of dust and dirt probably flew into the air. Researchers say this likely caused chemical acid rain and a warming of the Earth's atmosphere.