It was June 30 in 1908 when, we think, a fairly large space rock descended over eastern Siberia and caused a titanic explosion. Trees in a large region were knocked flat, all of them pointing away from a hypocenter near the Tunguska River. Thundering sounds were heard over vast areas. Shortly thereafter a series of "white nights" affected the northern hemisphere. It took a few years before the beleaguered Russian authorities could mount an expedition to the remote area, and decades more before a consensus came together. Read more about the Tuguska event, 104 years ago today.