Sooner or later, it was bound to happen. On June 30, 1908, Moscow escaped destruction by three hours and four thousand kilometers--a margin invisibly small by the standards of the universe.

So begins Rendezvous with Rama, a 1972 novel by Arthur C. Clarke in which mankind learns the hard way about the dangers posed by incoming asteroids. The 2077 impact in northern Italy that Clarke goes on to describe is fictional: the 1908 blast was real. The early morning of 30 June 1908 saw, in an area around the Stony Tunguska river, the most explosive cosmic impact in recent history, hundreds of times more powerful than the atomic weapons set off over Hiroshima and Nagasaki.

And yet, in part because it happened so far from civilization, and in part because it left no crater, it has not always been recognized as such. For decades it existed in a strange realm between science and pseudoscience, blamed on antimatter, black holes and alien spacecraft as easily as on a very fast bit of interplanetary refuse, and developing a mystique that has seen it associated with everything from energy drinks and rock bands to military missiles and The X-Files.

The approximate site of the blast's epicentre is now marked by a totem pole that researchers have dedicated to Agdy, the god of thunder in local mythology. Getting there is quite a trek, but the fascination of the site still draws an intermittent stream of scientists to the remote wilderness about 1,000 kilometres north of Lake Baikal; they leave offerings at the totem pole to commemorate the trek. In the years directly after the blast, though, no one came at all. The first researchers did not arrive until the 1920s.

That does not mean there was no significant contemporary evidence to bring to bear. Siberia was and is an empty place--but a blast which, had it happened over Chicago, would have been heard from Georgia to the Dakotas, still drew a lot of attention. In the days following the blast, A. V. Voznesenskij, the director of the Irkutsk magnetic and meteorological observatory near Lake Baikal, began collecting accounts that are vivid with detail (1). There are people being knocked off their feet, a man needing to hold onto his plough to avoid being swept away by a powerful wind, the feeling of great heat "as if my shirt had caught fire", herds of hundreds of reindeer being killed, trees set alight by the radiance of the fireball only for the flames to be snuffed out by the subsequent blast wave. And the reports are unequivocal on the source of the blast. G. K. Kulesh, head of a meteorological station at Kurensk, 200 kilometres from the epicentre, told Voznesenskij that:

There appeared in the northwest a fiery column ... in the form of a spear. When the column disappeared, there were heard five strong, abrupt bangs, like from a cannon, following quickly and distinctly one after another ... there had been a strong shaking of...