DESCRIPTION: It is a mystery on a global scale: five times in the past, life has been nearly extinguished, the vast majority of plants and animals annihilated in a geologic instant. What triggered these dramatic events? And what might they tell us about the fate of our world? "Mass Extinction: Life at the Brink" joins scientists around the globe as they unravel the mysteries of two of the most dramatic mass extinctions - the "K/T Extinction" that wiped out the dinosaurs, and "The Great Dying," which obliterated nearly 90% of all species on Earth. At first glance, these two extinctions could not look more different. A six-mile-wide asteroid spelled near-instant doom for the dinosaurs. And as new research covered in the film reveals, massive volcanic eruptions altered the chemistry of the atmosphere and ocean to trigger "The Great Dying." As different as they seem, these two extinctions share uncanny similarities and a message for today. Could the impact of human beings be just as devastating to the planet as a massive asteroid strike or volcanic eruptions?