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On 30 June 1908, an explosion ripped through the air above a remote forest in Siberia, near the Podkamennaya Tunguska river.
The fireball is believed to have been 50-100m wide. It depleted 2,000 sq km of the taiga forest in the area, flattening about 80 million trees.
The earth trembled. Windows smashed in the nearest town over 35 miles (60km) away. Residents there even felt heat from the blast, and some were blown off their feet.
The crash was followed by a noise like stones falling from the sky, or of guns firing
Fortunately, the area in which this massive explosion occurred was sparsely inhabited. There were no official reports of human casualties, though one local deer herder reportedly died after he was thrust into a tree from the blast. Hundreds of reindeer were also reduced to charred carcasses.
One eyewitness account said that "the sky was split in two, and high above the forest the whole northern part of the sky appeared covered with fire…
"At that moment there was a bang in the sky and a mighty crash… The crash was followed by a noise like stones falling from the sky, or of guns firing."
This "Tunguska event" remains the most powerful of its kind recorded in history – it produced about 185 times more energy than the Hiroshima atomic bomb (with some estimates coming in even higher). Seismic rumbles were even observed as far away as the UK.
And yet, over a hundred years later researchers are still asking questions about what exactly took place on that fateful day. Many are convinced that it was an asteroid or a comet that was responsible for the blast. But very few traces of this large extraterrestrial object have ever been found, opening the way for more outlandish explanations for the explosion.
The Tunguska region of Siberia is a remote place, with a dramatic climate. It has a long hostile winter and a very short summer, when the ground changes into a muddy uninhabitable swamp. This makes the area extremely hard to get to.
When the explosion happened, nobody ventured to the site to investigate. This was partly because the Russian authorities had more pressing concerns than sating scientific curiosity, says Natalia Artemieva of the Planetary Science Institute in Tucson, Arizona.
He found a large area of flattened trees, spreading out about 50km wide
Political strife in the country was growing – World War One and the Russian Revolution were just a few years away. "There were only some publications in local papers, not even in St Petersburg or Moscow," she says.
It was only a few decades later, in 1927, that a Russian team led by Leonid Kulik finally made a trip to the area. He had stumbled across a description of the event six years earlier and convinced Russian authorities that a trip would be worthwhile. When he got there, the damage was still immediately apparent, almost 20 years after the blast.
He found a large area of flattened trees, spreading out about 31 miles (50km) wide in a strange butterfly shape. He proposed that an extraterrestrial meteor had exploded in the atmosphere.
It puzzled him that there was no impact crater, or in fact, any meteoric remnants at all. To explain this, he suggested that the swampy ground was too soft to preserve whatever hit it and that any debris from the collision had been buried.
Kulik still hoped that he could uncover the remains, as he wrote in his 1938 conclusions. "We should expect to encounter, at a depth of hardly less than 25 metres, crushed masses of this nickeliferous iron, individual pieces of which may have a weight of one or two hundred metric tons."
Some suggested the Tunguska event could have been the result of matter and antimatter colliding
Russian researchers later said that it was a comet, not a meteor that caused the damage. Comets are largely made up of ice – not rock, like meteorites – so the absence of alien rock fragments would make more sense this way. The ice would have started to evaporate as it entered Earth's atmosphere, and continue to do so as it hit the ground.
But that was not the end of the debate. Because the exact identity of the explosion was unclear, strange alternative theories soon started to appear.
Some suggested the Tunguska event could have been the result of matter and antimatter colliding. When this happens, the particles annihilate and emit intense bursts of energy.
Another proposal was that a nuclear explosion caused the blast. An even more outlandish suggestion was that an alien spaceship crashed at the site on its search for the fresh water of Lake Baikal.
As you might expect, none of these theories stuck. Then, in a 1958 expedition to the site, researchers discovered tiny remnants of silicate and magnetite in the soil.
Further analysis showed they were high in nickel, a known characteristic of meteoric rock. The meteor explanation looked correct after all – and K. Florensky, author of a 1963 report on the event, was keen to put the more fantastical theories to rest:
They were more concerned with bigger asteroids that might cause global extinctions
"While I am aware of the advantages of sensational publicity in drawing public attention to a problem, it should be stressed that unhealthy interest aroused as a result of distorted facts and misinformation should never be used as a basis for the furtherance of scientific knowledge."
But that did not stop others coming up with even more imaginative ideas. In 1973 a paper was published in the reputable journal Nature, suggesting that a black hole collided into Earth to cause the explosion. This was quickly disputed by others.
Artemieva says ideas like this are simply a by-product of human psychology. "People who like secrets and 'theories' usually do not listen to scientists," she says. A huge explosion, coupled with a lack of cosmic remnants, is ripe for these kinds of speculations.
But she also says scientists must shoulder some responsibility, because they took so long to analyse the explosion site. They were more concerned with bigger asteroids that might cause global extinctions, just as the Chicxulub asteroid did. It wiped out most of the dinosaurs 66 million years ago.
In 2013 one team put a stop to much of the speculation of the earlier decades. Led by Victor Kvasnytsya of the National Academy of Sciences of Ukraine, the researchers analysed microscopic samples of rocks collected from the explosion site in 1978. The rocks had a meteoric origin. Crucially, the fragments they analysed were recovered from a layer of peat dating back to 1908.
Various gravitational interactions can make [asteroids] change their orbit more dramatically
The remnants had traces of a carbon mineral called lonsdaleite, which has a crystal structure almost like diamond. This particular mineral is known to form when a graphite-containing structure, such as a meteor, crashes into Earth.
"Our study of samples from Tunguska, as well as research of many other authors reveals meteorite origin of Tunguska event," says Kvasnytsya. "We believe that nothing paranormal happened at Tunguska."
The main problem, he says, is that researchers had spent too much time looking for large pieces of rock. "What was necessary was to look for very small particles," such as the ones his team studied.
But it is not a definitive conclusion. Meteor showers occur often. Many small ones might therefore sprinkle their remnants onto Earth unnoticed. Samples with meteoric origin could presumably come from one of these. Some researchers also cast doubt that the peat collected dates from 1908.
Even Artemieva says she needs to revise her models to understand the total absence of meteorites at Tunguska.
Still, in line with Leonid Kulik's early observations, today the broad consensus remains that the Tunguska event was caused by a large cosmic body, like an asteroid or comet, colliding with Earth's atmosphere.
Most asteroids have quite stable orbits, many of which are found in the asteroid belt between Mars and Jupiter. However, "various gravitational interactions can make them change their orbit more dramatically," says Gareth Collins of Imperial College London, UK.
Occasionally these rocky bodies can cross over into Earth's orbit which can put them onto a collision course with us. At the point one enters into our atmosphere and begins to fragment, it is known as a meteor.
What made the Tunguska event so dramatic was that it was an extremely rare case of what researchers call a "megaton" event – as the energy emitted was about 10-15 megatons of TNT, though even higher estimates have also been proposed.
This is also why the Tunguska event has been difficult to make full sense of. It is the only event of that magnitude that has happened in recent history. "That limits our understanding," says Collins.
Artemieva now says there are clear stages that took place, which she has outlined in a review to be published in the Annual Review of Earth and Planetary Sciences in the second half of 2016.
Most people think they come whaling in from outer space and leave a crater
First, the cosmic body entered our atmosphere at 9-19 miles per second (15-30km/s).
Fortunately, our atmosphere is good at protecting us. "It will break apart a rock smaller than a football field across," explains NASA researcher Bill Cooke, who leads NASA's Meteoroid Environment Office. "Most people think they come whaling in from outer space and leave a crater, and there's a big smoking piece of rock on the ground. The truth is kind of the opposite."
The atmosphere will generally break rocks up a few kilometres above the Earth's surface, producing an occasional shower of smaller rocks that, by the time they hit the ground, will be cold.
In the case of Tunguska, the incoming meteor must have been extremely fragile, or the explosion so intense, it obliterated all its remnants 8-10km above Earth.
This process explains the event's second stage. The atmosphere vaporised the object into tiny pieces, while at the same time intense kinetic energy also transformed them into heat.
"The process is similar to a chemical explosion. In conventional explosions, chemical or nuclear energy is transformed into heat," says Artemieva.
The intense heat resulted in shockwaves that were felt for hundreds of kilometres
In other words, any remnants from whatever entered Earth's atmosphere were turned into cosmic dust in the process.
If events unfolded this way, it explains the lack of large chunks of cosmic material at the site. "It is very difficult to find a millimetre-size grain in a big area. It is necessary to search in the peat," says Kvasnytsya.
As the object entered our atmosphere and broke apart, the intense heat resulted in shockwaves that were felt for hundreds of kilometres. When this airburst then hit the ground it flattened all the trees in the vicinity.
Artemieva suggests an enormous plume resulted from the updraught, which was then followed by a cloud, "thousands of kilometres in diameter".
But Tunguska's story is not over. Even now, some other researchers have proposed that we have been missing an obvious clue to explain the event.
In 2007 an Italian team suggested that a lake 5 miles (8km) north-north-west of the explosion's epicentre could be an impact crater. Lake Cheko, they say, did not feature on any maps before the event.
Luca Gasperini of the University of Bologna in Italy, travelled to the lake in the late 1990s, and says it is difficult to explain the origin of the lake in any other way. "Now we are sure it was formed after the impact, not from the main Tunguska body but of a fragment of the asteroid that was preserved by the explosion."
Any 'enigmatic' objects at the bottom of this lake could be easily recovered with minimal efforts
Gasperini firmly believes that a large piece of asteroid lies 33ft (10m) below the bottom of the lake, buried in sediment. "It would be very easy for Russians to get there and drill," he says. Despite heavy criticism of the theory, he still hopes someone will scour the lake for remnants of meteoric origin.
That Lake Cheko is an impact crater is not a popular idea. It is just another "quasi-theory" says Artemieva. "Any 'enigmatic' objects at the bottom of this lake could be easily recovered with minimal efforts – the lake is not deep," she says. Collins also disagrees with Gasperini's idea.
In 2008, he and colleagues published a rebuttal to the theory, stating that "unaffected mature trees" were close to the lake, which would have been obliterated if a large piece of rock had fallen close by.
Regardless of the details, the influence of the Tunguska event is still felt. Research papers on the subject continue to be published.
Today, astronomers also peer into the skies with powerful telescopes to look for signs that rocks with the potential to cause a similar event are heading our way, and to assess the risk that they pose.
When a Tunguska type event happens again, the overwhelming probability is that it will happen nowhere near human population
In 2013 in Chelyabinsk, Russia, a relatively small meteor around 62ft (19m) wide created visible disruption. This surprised researchers like Collins. His models had predicted it would not cause as much damage as it did.
"What's challenging is that this process of the asteroid disrupting in the atmosphere, decelerating, evaporating and transferring its energy to the air, is a very complicated process. We would like to understand it more, to better predict consequences of these events in future."
Chelyabinsk-sized meteors were previously believed to occur roughly every 100 years, while Tunguska-sized events had been predicted to occur once a millennium. This figure has since been revised. Chelyabinsk-sized meteors could be happening 10 times more frequently, says Collins, while Tunguska style impacts could occur as often as once every 100-200 years.
Unfortunately, we are and will remain defenceless against similar events, says Kvasnytsya. If another explosion like the Tunguska event took place above a populated city, it would cause thousands if not millions of casualties, depending where it hit.
But it is not all bad news. The probability of that happening is extremely small, says Collins, especially given the huge surface area of Earth that is covered in water. "When a Tunguska-type event happens again, the overwhelming probability is that it will happen nowhere near human population."
We may never find out whether the Tunguska event was caused by a meteor or comet, but in a way that does not matter. Either could have resulted in the intense cosmic disruption, which we are still talking about over a century later.
Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.
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