ScienceDaily (Apr. 1, 2010) — Some 13,000 years ago the Earth was struck by thousands of Tunguska-sized cometary fragments over the course of an hour, leading to a dramatic cooling of the planet, according to astronomer Professor Bill Napier of the Cardiff University Astrobiology Centre
that's why their prophecies are the best. Because you know - it will likely come again. Periodicity.

Was a giant comet responsible for a North American catastrophe in 11,000 BC?