Only one thing is certain about the morning of June 30th, 1908: Something big went “boom.” Other than that, no one is really sure what went down. The explosion absolutely destroyed 770 square miles of Eastern Siberia, but there were no casualties. Nor was there a crater, despite this being the largest recorded impact in world history—the equivalent of 15 to 30 megatons of TNT, a whopping 5.0 on the Richter scale and more than half of the largest nuclear blast. So what do we know about this strange explosion over Imperial Russia?

Both ethnic Russian settlers and native Evenki alike saw a blue light in the sky, reported to be as bright as the sun itself. Unlike the sun, however, this light moved across the early morning sky. Shortly after the object was first spotted, there was a flash and a loud explosion described as being like artillery guns. The shockwave broke windows hundreds of miles away and knocked observers off their feet.

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Across the Eurasian supercontinent, seismic stations picked up on the explosion. Atmospheric pressure changes registered as far away as Great Britain. The skies glowed for several nights afterward across Eurasia. Newspapers, expeditionists and tribesmen alike made note of the event, but no one really saw fit to investigate it until nearly a decade later.

For whatever reason, Imperial Russians didn’t have much interest in exploring what happened during what is now known as the Tunguska Event. It was left to their successors, the Soviets, to dig deeper. Leonid Kulik, a Russian mineralogist, made the first expedition to the area under the auspices of the Soviet Academy of Sciences. Guided by native hunters, his company refused to get too close to the impact site, fearing what they called “the Valleymen.”

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When Kulik finally brought together a new party and made it to the impact site, everyone was amazed. There was no crater. Instead, for an eight kilometer radius, all the trees had the branches burned off, but the trunks stood perfectly upright. Almost 40 years later, the total scorched area was found to be over 830 square miles in the shape of a butterfly.

Further expeditions found tons of tiny little potholes. Initially, Kulik thought these were tiny craters from the meteor breaking apart. They drained one of the resulting bogs to find old stumps at the bottom, ruling out the possibility that these strange formations were craters from a meteorite. Later expeditions found silicate and magnetite spheres in the soil as well as the trees. High levels of nickel in comparison to iron confirmed these spheres to be of extraterrestrial origin. Later studies found similarly strange combinations of metals, further lending support to the outer space hypothesis.
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The Tunguska event remains important in conspiracy lore, with some contending that nothing came from the sky at all. Some believe the ground itself exploded, while others believe ground zero is the site of a UFO crash or even a test run of Serbian-American inventor Nikola Tesla’s death ray. Another theory is that a black hole ran through our orbit or antimatter collided with the earth. For their part, the Evenki believe it was the work of Agda, their god of thunder.

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The only one of these theories with much credence to them is that the explosion came from the earth. Wolfgang Kundt, an astrophysicist with the University of Bonn, is among those who believe it was an explosion of natural gas out of kimberlite, a volcanic rock known to hold diamonds.

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Most of the scientific debate is about whether the Tunguska Event resulted from a comet or an asteroid. The glowing skies support the comet hypothesis, which was the preferred explanation of Soviet astronomers in the 1960s. Ľubor Kresák, a Slovak astronomer, later wrote that he believed it was a fragment of the Comet Encke, a periodic comet that orbits the Sun once every 3.3 years.

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The competing hypothesis is that it was an asteroid, which stands on somewhat shakier ground than the comet hypothesis. For one, there ought to be a crater. However, there’s also good explanation for why there wouldn’t be one: The asteroid may have exploded before impact. In this case, it wouldn’t have been a comet tail that caused the glow in the sky, but the explosion itself. Italian researchers found trace metals in nearby tree resin more commonly found in asteroids than comets.

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And yet… maybe there is a crater: Lake Cheko. In this scenario, a ten-meter chunk of rock was the sole remaining fragment and created a lake as a crater. Silt levels on the lake jibe with this theory, as they are consistent with what would be present in a 100-year-old lake. So, we may never know what really happened. This is Russia we are talking about, after all.