 In 1908 over Siberia, a terrifying event took place. A massive ball of fire was observed, brighter than the sun, and two huge explosions in a sonic boom were heard hundreds of miles away from the light flash. What had just taken place became known as the Tunguska event. The unprecedented event happened at 7.17am local time on the last day of June 1908. At that precise moment, an object brighter than the morning sun ripped through the atmosphere over Siberia. A witness trainload of passengers on the Trans-Siberian Railway stared in horror as the massive fireball roared through the clear blue skies at a phenomenal velocity of around one mile per second. The sonic boom given off by the sky invaders shook the railway track. The mysterious fiery object thundered north. The terrified train passengers watched with amazement as the overhead danger became fainter, and many of them looked out the windows of the carriages and watched the vapor trail with disbelief. The blast was 1,000 times more powerful than the atomic bombs dropped on Hiroshima and Nagasaki, and generated a shockwave that knocked people to the ground 100 miles from the epicenter. Eye witnesses recalled a brilliant fireball resembling a flying star plowing across the cloudless June sky at an oblique angle. One witness reports, At that moment I became so hot that I couldn't bear it, as if my shirt was on fire. I wanted to tear off my shirt and throw it down, but then the sky slammed shut. Something thump sounded, and I was thrown a few yards, he said. To many, this event, the biggest space impact of modern time, serves as a reminder of the continuing threat posed to our planet by objects from space. If the Tunguska impactor had exploded over a major city such as London, the death toll would have been up in the millions. It would have been total devastation, and this would have been the case had the object entered the atmosphere either two hours before, or two hours after. The trajectory would have indeed flattened the highly populated area. There was little scientific curiosity about the incident at the time, possibly due to the isolation of the Tunguska region. If there were any early expeditions to the sites, the records likely would have been lost during the subsequent chaotic years, World War I, the Russian Revolution of 1917, and the Russian Civil War. The first recorded expedition arrived at the scene more than 13 years after the event in 1921 to discover the sheer devastation that had taken place. 90 million trees and some 800 square miles of forest were completely flattened, enough to wipe London off the map. During the next 10 years, there were three more expeditions into the area, and no impact crater was ever found. It was suggested that it was an asteroid airburst event. In 1930, the British astronomer FJW Whipple suggested that the Tunguska body was a small comet, a comet also referred to as a dirty snowball. It is composed of dust and volatile such as water, ice, and frozen gases, and could have been completely vaporized by the impact with Earth's atmosphere, leaving no obvious traces. The comet hypothesis was further supported by the glowing skies observed across Europe for several evenings after the impact. In London, they were able to play cricket matches through the night, and newspaper print could be read in Paris with unaided light sources, possibly explained by the dust and ice that have been dispersed by the comet's tail across the upper atmosphere. This hypothesis gained a general acceptance among Soviet Tunguska investigators by the 1960s. In 1978, the Slovakian astronomer suggested that the body was a fragment of the comet Enkel. This is a periodic comet with an extremely short period of three years that stays entirely within the orbit of Jupiter. It is also responsible for the Beta Tiridus, an annual meteor shower with the maximum activity around the 28th and 29th of June. The Tunguska event coincided with the peak activity of that shower, and the approximate trajectory of the Tunguska object is consistent with what would be expected from a fragment of the comet Enkel. It is now known that bodies of this kind explode at frequent intervals tens to hundreds of kilometers above the ground. Military satellites have been observing these explosions for decades. Other theories in popular culture suggest a UFO got shot down and crash landed, much like a science fiction story. The Tunguska event is the strongest, but not the only example of an unobserved explosion event. Although significantly smaller, the 1930 Kotoka River event in Brazil was a similar explosion of a super bolide that left no clear evidence of an impact crater. Modern developments in infrasound detection have reduced the likelihood of undetected airbursts in the future. A smaller airburst occurred in the populated area in Russia on the 15th of February 2013 in Chelyabinsk in the rural district of Russia. The exploding meteor was an asteroid that measured to about 17 to 20 meters across with an estimated initial mass of 11,000 tons and inflicted over 1,200 injuries, mainly from broken glass falling from windows shattered by its shockwave. Over a century later, some still debate the cause and come up with different scenarios that could have caused the explosion, but the generally agreed-upon theory is that on the morning of June 30th, 1908, a large space rock of about 120 feet across entered the atmosphere of Siberia and then detonated into the sky. The estimate is that these events happen every 50 to 70 years, so it's likely that most of us will see more footage of a space invader on the news through social media. What do you guys think of the Tunguska event? Thanks for watching, and remember, the ways at which we arrive at knowledge is all that's wonderful in the discovery of these things themselves.