Enlarge / Some of the massive volcanic deposits that are part of the Siberian Traps. (credit: MIT) The end-Permian mass extinction was a big deal. It was the largest mass extinction event ever and occurred 252 million years ago. A whopping 90 percent of all marine species and around 70 percent of their terrestrial kin were killed off. Over the years, there have been numerous efforts to look into this massive, world-changing event.