Millions of tons of ash were thrown into the atmosphere – so much that it blocked out the sun worldwide. As far away as 2,500 miles away, 14 inches of ash blanketed the ground. Global temperatures plummeted 21 degrees. For months rain was a black, acidic sludge. Man was pushed to the edge of extinction, human population reduced to just a couple of thousand. Three quarters of all plants north of the equator were killed. Could it happen again? Yes, say geologists. In fact, we’re overdue.