Today’s Astronomy Picture of the Day is of the impact site for the 1908 Tunguska meteor, “the most powerful natural explosion in recent Earth history.” The meteor hit with a force of about 20 megatons, fortunately in a remote region of Siberia. Isaac Asimov pointed out that the latitude of the strike (60.96°N) was almost precisely that of St. Petersburg (59.93°N), so that if the meteor had arrived about 4hr 46m later, the second city of Imperial Russia would have been wiped out.