On June 1908, XNUMX, an explosion went through the air above a secluded forest in Siberia, near the Podkamenaya Tunguska River.
The fireball is believed to have been between 50 and 100 meters wide.
It devastated 2.000 square kilometers of taiga in the area, uprooting around 80 million trees.
The earth shook.
Windows were shattered in a nearby town more than 60 kilometers away.
Residents there felt the heat of the impact, and some were even knocked off their feet.
The impact was accompanied by a noise like stones falling from the sky or gunfire.
Fortunately, the area where this massive explosion took place was sparsely populated.
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There were no official reports of human casualties, although a local deer farmer was reportedly killed after being thrown into a tree by the explosion.
Hundreds of reindeer were also reduced to charred carcasses.
According to one witness, "the sky split open, and high above the forest, the entire northern part of the sky seemed to be covered with fire...".
"At that moment, there was a big bang in the sky and a loud impact sound was heard.
"The impact was accompanied by a noise like stones falling from the sky or gunfire."
This "Tungus explosion" remains the most powerful of its kind recorded in history - it produced about 185 times more energy than atomic bombs dropped on Hiroshima (according to some estimates, even more than that).
Seismic tremors have been observed as far away as Great Britain.
And yet, more than a hundred years later, researchers are still asking questions about what exactly happened on that fateful day.
Many are convinced that an asteroid or comet was responsible for the explosion.
But very little remains of this large extraterrestrial object have been found, opening the way to much more extravagant explanations for the explosion.
The Tunguska region in Siberia is a remote place with a drastic climate.
It has a long harsh winter and a very short summer, when the land turns into a muddy uninhabitable swamp.
That is why it is very difficult to reach that area.
When the explosion happened, no one went to the scene to investigate.
That's partly because the Russian authorities had much more important to do than satisfy scientific curiosity, says Natalia Artemiyeva of the Planetary Science Institute in Tucson, Arizona.
He found a huge area of downed trees, spanning an area 50 kilometers wide.
Political conflicts in the country were intensifying - the First World War and the Russian Revolution were only a few years apart.
"There were only a few articles in the local papers, not even in St. Petersburg or Moscow," she says.
It wasn't until several decades later, in 1927, that a Russian team led by Leonid Kulik finally traveled to the area.
He came across the description of the event six years later and convinced the Russian authorities that the trip was necessary.
When he finally got there, the devastation was still very evident, almost 20 years after the explosion.
He found a huge area of fallen trees, spreading over a territory 50 kilometers wide in an unusual butterfly-like shape.
He assumed that a meteor from outer space had exploded in the atmosphere.
He was surprised that there was no impact crater or, indeed, any meteor debris at all.
To explain this, he suggested that the marshy ground was too soft to preserve whatever had hit it and that the debris from the impact remained buried deep within it.
Kulik still hoped to find the remains, as he wrote in his 1938 conclusions.
"We should expect to encounter, at a depth of scarcely less than twenty-five meters, a crushed mass of that nickel-plated iron, individual pieces which might weigh as much as one hundred or two hundred metric tons.
Some have suggested that the Tungun explosion could be the result of matter and antimatter colliding
Russian researchers later announced that the destruction was caused by a comet, not a meteor.
Comets are mostly made of ice - not rock, like meteorites - so the absence of alien rock fragments would make more sense.
The ice would begin to evaporate immediately upon entering the Earth's atmosphere and continue to do so when it hits the ground.
But that was not the end of this discussion.
Because the exact identity of the explosion was unknown, unusual alternatives quickly began to spring up.
Some have suggested that the Tungun explosion could be the result of matter and antimatter colliding.
When this happens, the particles disintegrate and emit powerful bursts of energy.
Another suggestion was that the source was a nuclear explosion.
An even more extravagant suggestion was that an alien spacecraft had crashed at the site while searching for the drinking water of Lake Baikal.
As you might guess, none of those theories caught on.
Then, in a 1958 expedition to the actual scene, researchers discovered tiny remains of silicate and magnetite in the soil.
Additional analyzes showed a high concentration of nickel, a known constituent of meteoric rock.
The meteor explanation seemed right after all - and K.
Florenski, the author of a report on the event from 1963, was determined to bury all the more fantastic theories once and for all:
They were more concerned about larger asteroids that could cause global extinctions
"Although I am aware of the benefits of sensationalist publicity when drawing public attention to a problem, it must be emphasized that unhealthy interest expressed as a result of distorted facts and misinformation must never be used as a basis for advancing scientific knowledge."
But that didn't stop others from coming up with even more imaginative ideas.
In 1973, a study was published in the respected journal Nature that suggested that a black hole collided with the Earth and caused an explosion.
Others quickly debunked that theory.
Artemieva says that such ideas are simply a byproduct of human psychology.
"People who like secrets and 'theories' usually don't listen to scientists," she says.
The huge explosion, combined with the absence of cosmic debris, is fertile ground for this kind of speculation.
But she also says that scientists have to take some responsibility, because it took them a long time to analyze the explosion site.
They were more concerned about larger asteroids that could cause global extinctions, as the Chickslab asteroid did.
It wiped out most of the dinosaurs 66 million years ago.
In 2013, one team put an end to most of the speculation from the previous decades.
Led by Viktor Kvasnytsia of the National Academy of Sciences of Ukraine, the researchers analyzed microscopic rock samples collected from the site of the 1978 explosion.
The rocks were of meteoric origin.
Crucially, the fragments they analyzed were extracted from a peat layer originating in 1908.
Various gravitational interactions can cause asteroids to change their orbits more drastically
The remains contained traces of a carbon mineral called lonsdellite, which has a crystal structure almost like diamond.
This particular mineral is known to form when a structure containing graphite, such as a meteor, hits the Earth.
"Our study of samples from Tunguska, as well as the research of many other authors, points to a meteoric origin of the Tunguska explosion," says Kvasnicija.
"We believe that nothing paranormal happened in Tunguska."
The main problem, he says, is that researchers have spent too much time looking for large chunks of rock.
"We should have been looking for very small particles," like the one his team studied.
But that is not the final conclusion.
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Meteor showers happen often.
Many smaller such rains could have left their remnants on Earth unnoticed.
Samples with a meteoric origin could have come from just one of them.
Some researchers have also expressed doubt that the collected peat actually dates back to 1908.
Even Artemieva says she has to revise her models to understand the complete absence of the Tunguska meteorite.
Nevertheless, in accordance with Leonid Kulik's early observations, today the broadest consensus remains that the Tungun explosion was caused by a huge cosmic body, such as an asteroid or comet, colliding with the Earth's atmosphere.
Most asteroids have stable orbits and can be found in the asteroid belt between Mars and Jupiter.
However, "various gravitational interactions can cause asteroids to change their orbits more drastically," says Gareth Collins of Imperial College London.
These rocky bodies can occasionally enter Earth's orbit, which can put them on a collision course with us.
At the point where one of them enters our atmosphere and begins to break up, it becomes known as a meteor.
What made the Tunguska explosion so dramatic was that it was an extremely rare event of what researchers call a "megaton" event - because the energy released was roughly 10-15 megatons of TNT, although higher estimates have been made.
This is also the reason why it is so difficult to fully understand the Tunguska explosion.
It is the only event of this magnitude that has occurred in recent history.
"It limits our understanding," says Collins.
Artemieva now says there are obvious stages that took place, which she highlighted in a review published in the 2016 Annual Review of Earth and Planetary Sciences.
Most people think that they tumble out of open space and leave behind a crater
First of all, the cosmic body entered our atmosphere at a speed of 15-30 kilometers per second.
Fortunately, our atmosphere protects us well.
"It will scatter the rock over a width smaller than a football field," explains NASA researcher Bill Cook, who heads NASA's Office of the Meteoroid Environment.
"Most people think that they tumble out of open space and leave behind a crater, leaving a huge smoldering chunk of rock in the ground." The truth is just the opposite."
The atmosphere will generally break up the rocks a few kilometers above the Earth's surface, producing an occasional shower of smaller rocks, which, by the time they hit the ground, are already cold.
In the case of the Tunguska explosion, the falling meteor must have been extremely fragile, or the explosion was so powerful that it destroyed all debris eight to 10 kilometers above the Earth.
This process explains the second stage of events.
The atmosphere scattered the object into tiny pieces, while at the same time the strong kinetic energy turned them into heat.
"That process is similar to a chemical explosion. In conventional explosions, chemical or nuclear energy is converted into heat," says Artemjieva.
The intense heat resulted in a shock wave that was felt hundreds of kilometers away
In other words, any remnants of what entered Earth's atmosphere turned into cosmic dust in the process.
If the event played out this way, it explains the absence of large chunks of cosmic material at the site.
"It is very difficult to find millimeter-sized particles in such a large area. That is why it is necessary to search for peat," says Kvasnicija.
When the object flew into our atmosphere and disintegrated, the intense heat resulted in a shock wave that was felt hundreds of kilometers away.
When this gust of air then hit the ground, it knocked down all the trees nearby.
Artemieva suggests that a huge column of smoke was created by an upward thrust of air, which was then followed by a cloud, "thousands of kilometers in diameter".
But the story of the Tunguska explosion does not end there. Even today, some other researchers suggest that we have missed an obvious clue that would explain this event.
In 2007, the Italian team hypothesized that the lake eight kilometers northwest of the epicenter of the explosion could have been a crater created by the collision.
Lake Ceko, they say, did not exist on any map before this event.
Luca Gasperini of the University of Bologna, Italy, traveled to the lake in the late XNUMXs, and says it is difficult to explain its origin in any other way.
"We are now certain that it was formed after the collision, not from the main Tunguska body, but from an asteroid fragment that was preserved by the explosion."
Any "enigmatic" objects at the bottom of this lake could be easily retrieved with minimal effort
Gasperini firmly believes that a large piece of asteroid lies ten meters below the bottom of the lake, buried in sediment.
"It would be very easy for the Russians to get there and start drilling," he says.
Despite fierce criticism of this theory, he still hopes that someone will search the lake for remnants of meteoric origin.
That Lake Čeko is an impact crater is not a very popular idea.
It's just another "quasi-theory", says Artemieva.
"Any 'enigmatic' objects at the bottom of this lake could easily be retrieved with minimal effort - the lake is not very deep," she says.
Collins also disagrees with Gasperini's idea.
In 2008, he and colleagues published a refutation of the theory, pointing out that there are "intact older trees" near the lake, which would have been completely destroyed if some large piece of rock had fallen nearby.
Regardless of the details, the impact of the Tunguska explosion is still being felt.
Research studies on this topic continue to be published.
Today, astronomers are also peering into the sky with powerful telescopes for signs that rocks with the potential to cause a similar explosion are headed our way and to assess what kind of risk they pose.
When an event similar to the Tunguska explosion happens again, there is a huge probability that it will not happen anywhere near the human population
In Chelyabinsk, Russia, in 2013, a relatively smaller meteor of about 19 meters left visible consequences. This surprised researchers like Collins.
His models predicted that he would not cause as much damage as he did.
"The biggest challenge is that this process, when an asteroid damages the atmosphere, slows down, then begins to disintegrate and turns its energy into air, is a very complicated process.
We would like to understand it better, in order to better predict the consequences of those events in the future."
Meteors the size of the one in Chelyabinsk were previously thought to occur roughly every 100 years, while Tunguska-type events occur once a millennium.
This assessment has since been revised.
Meteors the size of the one in Chelyabinsk could be 10 times more common, Collins says, while Tunguska could happen as often as every 100 to 200 years.
Unfortunately, we are still helpless against similar events, says Kvasnicija.
If another Tunguska-like explosion were to occur over a populated city, it would lead to thousands if not millions of casualties, depending on where it hits.
But not all news is bad.
The likelihood of that happening is extremely low, says Collins, especially when you consider that a huge amount of the Earth's surface is covered in water.
"When an event similar to the Tunguska explosion happens again, there is a huge probability that it will not happen anywhere near the human population."
We may never know whether the Tunguska explosion was caused by a meteor or a comet, but in some ways it doesn't even matter.
Both could have led to a violent cosmic upheaval that we're still talking about more than a century later.
Melissa Hogenboom writes articles for BBC Earth. She is @melissasuzanneh on Twitter.
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