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The Tunguska event was an explosion that occurred near the Podkamennaya (Under Rock) Tunguska River in what is now Krasnoyarsk Krai of Russia, at 7:17 AM on June 30, 1908. The event is sometimes referred to as the Great Siberian Explosion.

The explosion was probably caused by the airburst of an asteroid or comet 5 to 10 kilometers (3–6 mi) above the Earth's surface. The energy of the blast was later estimated to be between 10 and 20 megatons of TNT, which would be equivalent to Castle Bravo, the most powerful nuclear bomb ever detonated by the US. It felled an estimated 80 million trees over 2,150 square kilometers (830 sq mi). An overhead satellite view centered at 60°55′N 101°57′E (near ground zero for this event) shows an area of reduced forest density, with a fully visible irregular clearing of somewhat less than one square kilometer in area.

In recent history, the Tunguska event stands out as one of the rare large-scale demonstrations that a full doomsday event is a real possibility for the human race.

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It must be stressed that the odds of impact are now known to be very low. Hence, the possible effects of an impact are largely irrelevant.

However, the initial reports resulted in widespread discussion on many Internet forums, including armchair speculation about exactly where Apophis (then known only as 2004 MN4) would hit and what would happen when it did.

NASA initially estimated the energy that Apophis would have released if it impacted Earth as the equivalent of 1480 megatons of TNT. A more refined later NASA estimate was 880 megatons. The impacts which created the Barringer Crater or caused the Tunguska event are estimated to be in the 10-20 megaton range. The 1883 eruption of Krakatoa was the equivalent of roughly 200 megatons.

The exact effects of any impact would have varied based on the asteroid's composition, and the location and angle of impact. Any impact would have been extremely detrimental to an area of thousands of square kilometres, but would have been unlikely to have long-lasting global effects, such as the initiation of an impact winter.

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