The Tunguska Event

On June 30, 1908, a giant fire ball flew over the vast territory of Central Siberia. The flight was accompanied by a loud sound, bright lights and it ended with a powerful explosion. The blast knocked down a forest in a radius of 40 kilometers. For several nights an unusual light phenomena called "bright nights summer 1908” were observed.
There are many theories about this event. Most of the people consider that it was big meteorite. Since 1927 scientists tried to find traces of explosion. However, the usual meteor crater has not been found so far. Other scientists noticed that area of fallen timber has a characteristic form of "Butterfly". So, the explosion was not in the collision of a body with the earth's surface, but before that in the air at an altitude of 5-10 kilometers. Some people think that it was Alien spaceship crash.
Anyway, the Tunguska event is the largest impact event over land in Earth's recent history. Estimates of the energy of the blast are about 10–15 megatons of TNT (42–63 PJ). It is roughly equal to the United States' Castle Bravo thermonuclear bomb tested on March 1, 1954. It is also about 1,000 times as powerful as the atomic bomb dropped on Hiroshima.