The most latest and most well known mass extermination is the one which ended the historic period of the dinosaurs, the main suspect behind this particular catastrophe is a cosmic effect from an asteroid or comet, more than a billion times more than the atom bombs that wrecked Hiroshima and also Nagasaki.
Resources:
http://news.discovery.com/animals/dinosaurs/asteroid-impact-that-killed-the-dinosaurs-130207.htm
