The Tunguska event

A huge explosion over Siberia on 30 June 1908 that Evenki reindeer herders and a Russian villager saw and heard. NASA calls it an asteroid impact; the idea of a crashed alien ship gained ground after a 1946 science fiction story.

On 30 June 1908, NASA's history office says, an asteroid entered the atmosphere and exploded over Siberia. Wikipedia puts the blast at 3 to 50 megatons of TNT, in Yeniseysk Governorate of the Russian Empire, close to the Podkamennaya Tunguska River, today part of Krasnoyarsk Krai.

NASA describes what the few Evenki reindeer herders who saw it reported: a fireball with a trail of smoke, a flash that outshone the Sun, then a noise like thunder. Those nearest said the blast threw them into the air and left them unconscious, and their homes were damaged or wrecked. So few people lived there that very few died, NASA says, though herds of reindeer were reported killed. Witnesses also told of great forest fires and of trees knocked flat for miles, and people farther off saw a tall column of smoke.

In 1967 Tomsk University printed eyewitness reports compiled by V. G. Konenkin. One came from Ivan Balakshin, born in 1897 and living in Erbogachen when interviewed, who in 1908 lived in the village of Zhdanova in the Katanga district. He said he looked west and saw fire leap up about as high as a tree, followed by smoke that rose above it and soon vanished. He recalled three explosions, and the noise made the window glass in the houses rattle.

In 2001 BBC News reported that an Italian team had put newly translated eyewitness accounts together with seismic records and a fresh survey of the site, and concluded the object was most likely a low-density asteroid. BBC Earth in 2016 listed stranger ideas, among them a nuclear blast and, further out still, an alien craft that came down while looking for fresh water in Lake Baikal. Wikipedia says the spaceship idea became popular after Alexander Kazantsev, a Russian author of science fiction, published his short story "Explosion" in 1946, and that fiction has since offered many other explanations.