At 7:14 a.m. on June 30, 1908, a giant explosion shook central Siberia. Witnesses close to the event described seeing a fireball in the sky, as bright and hot as another sun. Millions of trees fell and the ground shook. Although a number of scientists investigated, it is still a mystery as to what caused the explosion.
Although the Richter scale was not yet invented, the explosion is estimated to have created the effects of a magnitude 5.0 earthquake, causing buildings to shake, windows to break, and people to be knocked off their feet even at 40 miles away.
The blast, centered in a desolate and forested area near the Podkamennaya Tunguska River in Russia, is estimated to have been a thousand times more powerful than the bomb dropped on Hiroshima. The explosion leveled an estimated 80 million trees over an 830 square-mile area in a radial pattern from the blast zone. Dust from the explosion hovered over Europe, reflecting light that was bright enough for Londoners to read at night by it.
Because of the blast zone's remote location and the intrusion of worldly affairs (World War I and the Russian Revolution), it wasn't until the 1920s that the first scientific expedition was sent to examine the area. Assuming that the blast had been caused by a falling meteor, the expedition expected to find a huge crater as well as pieces of the meteorite. They found neither. Later expeditions also were unable to find credible evidence to prove the blast was caused by a falling meteor.
In the decades since this huge explosion, scientists and others have attempted to explain the cause of this mysterious event. The most commonly accepted scientific explanation is that either a meteor or a comet entered the Earth's atmosphere and exploded a couple of miles above the ground (this explains the lack of impact crater).
Other explanations have ranged from the possible to the ludicrous, including a natural gas leak escaped from the ground and exploded, a UFO spaceship crashed, the effects of a meteor destroyed by a UFO's laser in an attempt to save Earth, a black hole that touched Earth, and an explosion caused by scientific tests done by Nikola Tesla.
Over a hundred years later, the Tunguska Event remains a mystery. However, if the blast was caused by a comet or meteor entering the Earth's atmosphere, it poses the serious possibility that in the future a similar meteor could once again enter Earth's atmosphere, but this time, land on a populated area. The result would be catastrophic. Researchers continue to study the area to find answers to their many questions.
Source:
http://history1900s.about.com/od/1900s/qt/Tunguska.htm
