‘Early in the morning on June 30, a very unusual phenomenon was observed here in a village in Siberia. High above the horizon, the peasants saw an object shining very brightly; it was too bright for the naked eye. Low on the horizon, in the same direction as the luminous body, a small black cloud could be seen. When the bright object approached the ground, it seemed to be crushed to dust. In its place a vast cloud of black smoke formed, and a loud explosion, as if from an avalanche of large stones, was heard. Buildings shook, and a forked tongue of flame burst upward through the cloud. The villagers ran into the street in terror. Old women wept; everyone thought that the end of the world was upon them.’—Summary of a report that appeared in the newspaper Sibir, Irkutsk, Russia, on July 2, 1908.

LITTLE did those villagers realize that an object from heaven had just exploded over their heads.

On June 30, 1908, an asteroid or a chunk of a comet estimated to be less than 300 feet [100 m] across roared into the atmosphere and exploded some five miles [10 km] above the largely unpopulated Tunguska region of Siberia, as mentioned in the introduction. The blast, estimated at 15 megatons, devastated an area of 800 square miles [2,000 sq km], knocking down trees, starting fires, and killing reindeer. How many people would have died if ground zero of that explosion had been a densely populated area?