At around 7:17 a.m. local time, Tungus natives and Russian settlers in the hills northwest of Lake Baikal observed a column of bluish light, nearly as bright as the Sun, moving across the sky. About 10 minutes later, there was a flash and a sound similar to artillery fire. Eyewitnesses closer to the explosion reported the sound source moving east to north. The sounds were accompanied by a shock wave that knocked people off their feet and broke windows hundreds of kilometres away. The majority of witnesses reported only the sounds and the tremors, and not the sighting of the explosion. Eyewitness accounts differ as to the sequence of events and their overall duration.The explosion registered on seismic stations across Eurasia. In some places the shock wave would have been equivalent to an earthquake of 5.0 on the Richter scale. It also produced fluctuations in atmospheric pressure strong enough to be detected in Great Britain. Over the next few days, night skies in Asia and Europe were aglow.
One study "suggests that a chunk of Comet 2005NB56 caused the 5–10 megaton fireball, bouncing off the atmosphere and back into orbit around the sun." The scientists involved in the study claim that the object that caused the event will pass close to Earth again in 2045
In 1989, Serge J.D. D'Alessio and Archie A. Harms suggested that some of the deuterium in a comet entering the Earth's atmosphere may have undergone a nuclear fusion reaction, leaving a distinctive signature in the form of carbon-14. They concluded that any release of nuclear energy would have been almost negligible. Independently, in 1990, César Sirvent proposed that a deuterium comet, i.e., a comet with an anomalous high concentration of deuterium in its composition, could have exploded as a natural hydrogen bomb, generating most of the energy released.
In 1973, Albert A. Jackson and Michael P. Ryan, physicists at the University of Texas, proposed that the Tunguska event was caused by a small (around 1017 kg to 1019 kg) black hole passing through the Earth. This hypothesis is flawed, as there was no so-called exit event — a second explosion occurring as the black hole, having tunnelled through the Earth, shot out the other side on its way back into space.
Alien spaceship crash
A number of theories based on UFOs have claimed that the Tunguska event was the result of the activities of extraterrestrial beings, including an exploding alien spaceship or even an alien weapon going off to "save the Earth from an imminent threat". These claims appear to originate from a science fiction story "The Explosion" written by the Soviet engineer Alexander Kazantsev in 1946, in which a nuclear-powered Martian spaceship, trying to land on the Earth, meets with a disaster and blows up in mid-air.
Astrophysicist Wolfgang Kundt has suggested the Tunguska event was caused by the sudden release and subsequent explosion of 10 million tons of natural gas from within the Earth's crust. The similar verneshot hypothesis has also been suggested as a possible cause of the Tunguska event Source