The most catastrophic extinction event in the history of the earth was the Permian Extinction 250 million years ago. Scientists believe this was caused by a massive half million year eruption in the Siberian traps that lead to wave after wave of toxic gases devastating life on the planet beginning with sulfur dioxide followed by carbon dioxide, hydrogen sulfide and methane. Yet, some animals found sanctuaries where they could survive.
 

So far, humans have been lucky to not experience a catastrophe on a global scale. Though they are the most powerful species on this planet, it's easy to forget that the forces that wiped out many successful species in the past are still at work.