I've never heard of Tunguska either.
::high fives fellow ignoramous::
The most prominent theory about Tunguksa (according to wikipedia) is that a meteor (about 30 feet across) exploded before it struck the earth. The blast pattern in the trees is butterfly shaped, which is consistent with a high powered blast that occurs above ground with initial forces going straight down, then spreading out horizontally after hitting the earth.
My favorite theory is that Tesla set it off:
It has also been suggested, by Oliver Nichelson, that the Tunguska explosion was the result of an experiment by Nikola Tesla at his Wardenclyffe Tower, performed during one of Robert Peary's North Pole expeditions. It is claimed that Tesla sent a communication to Peary advising him to be on the alert for 'unusual auroral phenomena' encountered as he attempted to reach the North Pole. However, by the time of the Tunguska event most work at Wardenclyffe had already ended and the site was mostly abandoned. In addition, it is by no means apparent how the small energy input at Wardenclyffe could be responsible for such a large energy output elsewhere. Also this event happened on June 30, 1908 and Peary didn't leave New York for the North Pole until July 6, 1908.
The Nichelson theory is that Peary would encounter a destruction field when he did reach the North Pole so having the Tunguska event take place before Peary departed does not contradict the theory. Though Tesla had lost major research funding by 1908, his Wardenclyffe facility was still operational in 1908. It was not until 1917 that it was torn down. The theory is that the lack of funding may have caused Tesla to use his transmitter in this dramatic fashion in order to continue his power transmission investigations.
The large amount of energy required to cause such an event could easily be achieved by the process of electrical resonance in which power could be built up over a period of days. Electrical resonance was a process well known at that time. This power build up over a period of time corresponds to the "bright nights" reported over Europe days prior to the explosion.
There are other theories such as a naturally occuring hydrogen bomb from a deuterium rich meteor.