Tunguska event

Tunguska event is the name for a very large mid-air explosion that occurred on 30 June 1908 in Siberia. Most eyewitnesses talk about one or more explosions that happened around 7:15 a.m. local time. The cause of these explosions is unknown, but a meteorite impact has been suggested as a likely cause.  About  around the place where the event happened, trees were uprooted. In Wanwara, a small settlement, about  away, windows and doors were smashed. It is estimated, that in an area of about , the trees were uprooted. Over  away, bright light was seen, and an earthquake was felt. 

There are no reports of people who have died. This is attributed to the fact that almost no people lived in the area, at the time. Depending on the source, there were one or two casualties.Ian Ridpath: "Tunguska: the final answer". Published in: New Scientist. 11 August 1977, Vol. 75, No. 1064, page 346.C. Gritzner: "Human Casualties in Impact Events". Published in: WGN, Journal of the International Meteor Organization. October 1997, Vol. 25:5, pp 222-226.

Today, it is believed that the event happened about  above ground, at 0:14 UTC (7:14 a.m. local time).

 Energy involved 
What happened is called air burst today. At the time, the energy involved was estimated to be between 10 and 30 megatons of TNT, depending on the height in which the air burst happened. Simulations that have been done with modern computers have found that the energy involved was probably between 3 and 5 megatons of TNT.  Taking 15 megatons, this would mean that the energy involved was 1.000 times that of the atomic bomb dropped on Hiroshima, in 1945. The shock wave is considered to be that of an earthquake with a magnitude of 5.0 on the Richter scale. The explosion registered at seismic stations across Eurasia, and air waves from the blast were detected in Germany, Denmark, Croatia, and the United Kingdom – and as far away as  modern-day Jakarta, Indonesia, and Washington, D.C.  For a few days after the event, night skies in Asia and Europe were aglow. There are reports of brightly lit photographs being successfully taken at midnight (without the aid of flashbulbs) in Sweden and Scotland. One explanation of this glow is that light passed through ice particles at a high altitude, that had formed at low temperatures during the explosion. Decades later, Space Shuttles reported similar findings. In the United States, a Smithsonian Astrophysical Observatory program at the Mount Wilson Observatory in California observed a months-long decrease in atmospheric transparency consistent with an increase in suspended dust particles.  

For these reasons, the Tunguska event is considered the largest impact event on Earth in recorded history. 

 Expeditions to the area 
At first, there was no interest in the event. Over a decade after the event, Leonid Kulik, a Russian mineralogist led an expedition to the area. In his expedition of 1921 and 1922, he only got to Kansk, which is  away from the "epicenter" of the event.  In 1927, Kulik reached the region where the event happened, in 1938 he ordered aerial photos be taken.

Kulik's expedition is the first recorded expedition to the area. Since then, there were many expeditions; over 1,000 articles have been published on the event; most of them are in Russian language.

There were two other expeditions to the area: one in 1958, the other in 1963.

Theories
Today, there is no definite explanation for what caused the explosions, or for what happened. There are different theories, though.

Meteorite explosion
A meteorite hit the area. The most probable is an asteroid or comet of low density, and a diameter of between  and . The object exploded in a height of 7 to 14 km above ground; for this reason there is no impact crater. The problem with this theory is that meteorites of this type usually do not reach low levels of the atmosphere. Meteoroids, which contain more iron have a higher chance of reaching low levels of the atmosphere, but they do not cause such devastating explosions. 

 Lake Cheko 
Lake Cheko is a small freshwater lake, about  from the center of the event. The lake does not show on maps of the area which are older than about 1928. Before 1908, it seems to be unknown. The lake is shaped roughly like a rectangle; it is 708 metres long, 364 m wide, and about 50 m deep. In 1999, Luca Gasperini did a study on the sediments in the lake, and found that the lake predates the event of 1908.Geophysical/sedimentological study of a lake close to the epicenter of the great 1908 Siberian (Tunguska) Explosion , NGF Abstracts and Proceedings, no. 1, 2001 : « Although the morphology of the lake is compatible with an impact origin, several sedimentological and biological proxies indicate that its formation pre-dates the 1908 event. » In 2007, National Geographic cites him as he states that it is possible the lake formed as an impact crater.This is argued upon due to the spread of trees, that were destroyed from the mostly agreed upon meteorite explosion.

Geophysical theories: Volcanic eruption
The other theory is that the event has a geophysical cause: About 10 million tons of natural gas exploding would have the effect that was observed. The problem with this theory is that eyewitnesses spoke of a very bright light that could be seen from very far away. A natural gas explosion does not produce a bright light. 

References

Category:Natural disasters in Asia
Category:1908
Category:Siberia
Category:20th century in Russia
Category:Impact events