"While they have not put forward an alternative explanation for the explosi on, outlandish theories in the past have included a massive volcanic erupti on, a comet mainly composed of ice not solid space rock, a black hole colli ding with Earth and even aliens shooting down a meteor from a UFO in order to save Earth."
Mystery Deepens Over World's Biggest Explosion In Russia Daily Mail - January 23, 2017
--
formatting link