In late 1908, the scientific community in St. Petersburg and Moscow was galvanized by vague reports filtering out of Siberia, telling of a gigantic, mysterious explosion that summer that had been witnessed only by a handful of native Evenki tribesmen and Russian settlers.

According to eyewitness reports gathered later by Leonid Kulik, at 7:17 am on June 30, 1908, a column of bright blue light became visible in the sky above central Siberia, followed by a huge explosion near Tunguska, a remote region located in the taiga (pine forest) northwest of Lake Baikal.

The explosion rocked the earth, leveled gigantic swaths of forest, and filled the sky with blinding light, according to one Evenki native, who later remembered: “Then I saw a wonder: trees were falling, the branches were on fire, it became mighty bright, how can I say this, as if there was a second sun, my eyes were hurting, I even closed them.” A Russian settler reported that the explosion blew out windows in buildings up to 40 miles away from the impact, accompanied by intense heat: “At that moment I became so hot that I couldn't bear it, as if my shirt was on fire… hot wind raced between the houses, like from cannons…” The explosion clearly occurred some distance above the ground, with one eyewitness recalling that “the sky split in two and fire appeared high and wide over the forest.”

The effects of the Tunguska Event were breathtaking: the explosion, measuring between 10 and 20 megatons (somewhere between 435 and 870 times the force of the Nagasaki bomb), leveled approximately 80 million trees over an area of 800 square miles, or around half a million acres. No surprise, the simple peasant folk who witnessed it assumed the worst, with one newspaper reporting that “women cried, thinking it was the end of the world.” Later, particulate matter in the atmosphere produced stunning sunsets around the world.

It’s still not clear what caused the Tunguska Event, but based on the eyewitness reports and butterfly-shaped blast pattern, most scientists agree it must have resulted from a meteor or comet exploding about six miles above the ground. The latest theory suggests the object measured around 100 feet across and weighed over 600,000 tons; that’s about three times the size of the world’s largest ocean liners, which weigh in at around 225,000 tons. Interestingly, many eyewitnesses described not just one but a series of explosions, raising the possibility that the event was caused by a string of meteors or parts of a disintegrating comet hitting the earth in succession.

However scientific expeditions led by Kulik beginning in 1921 were unable to recover any fragments of the meteor or comet, and subsequent investigations (including an Italian mission which may have found a crater caused by the explosion in 2007) have also come up empty-handed. Predictably, this has given rise to all sorts of bizarre conspiracy theories, including, yes, an extraterrestrial visitation. Why aliens would want to destroy a remote part of Siberia, and what they have against pine trees, remains unclear.