An enormous Supervolcano which houses an incredibly huge magma chamber deep underground which is several miles across and is just waiting to erupt and blow our sorry arses into the next world. Will most likely happen when some 3 billion people are having a coffee break or having a toilet break.
Joe: The sky is dark, its snowing in the middle of july and my home has collapsed from mountains of volcanic ash crushing my piles of porno mags.
Mike: oh well, Yellowstone has erupted. The end of the world has to start somewhere.