The Tunguska explosion
The account
An alien spacecraft exploded over Siberia, which would explain why there is no crater and no meteorite fragments.
What investigators found
The most widely accepted explanation is the airburst of an asteroid or comet a few tens of meters across, several kilometers above the ground.
The story
On the morning of June 30, 1908, a gigantic explosion shook a remote region of Siberia near the Podkamennaya Tunguska River. About 80 million trees were flattened across an area of more than 2,000 square kilometers. People tens of kilometers away were knocked to the ground, and the skies over Europe stayed strangely bright for several nights.
The first scientific expedition did not arrive until 1927, and it found no crater. That opened the door to theories: an alien spacecraft, antimatter, even experiments by Nikola Tesla. Scientists today agree that an object from space exploded in the air before reaching the ground, which explains the trees knocked down in a fan pattern and the missing hole.