On June 30, 1908, an enormous and apparently spontaneous aerial explosion leveled 80 million Siberian trees near the Tunguska River. Eyewitness descriptions would make the hairs stand up on the back of your neck: a column of blue light moving across the heavens, followed by the "sky split[ting] in two," intense heat, a wall of thunder.

The blast, estimated at 10-15 megatons, has a number of fanciful explanations:

• Perhaps it was a naturally-occurring hydrogen bomb, resulting from the impact of a comet with an unusually large amount of deuterium. Alas, there is virtually no science behind this hypothesis.
• It could have been a black hole passing through the earth — except there was no exit event. A number of science fiction authors have embraced this hypothesis in literature, but fiction is likely all it is.
• Some have speculated it could have been caused by an extraterrestrial chunk of antimatter. But the event left mineral traces, not gamma rays, so this hypothesis is out.

I will not even go into the UFO hypotheses.

Scientists have come to the tentative consensus that the Tunguska event was actually caused by some sort of space object, perhaps a comet or meteor, that entered the atmosphere and exploded in mid-air. It's not an exotic an explanation as some of the ones that have been proposed over the past century, but it is still eye-goggling awesome: It came from outer space!

No crater or large fragments remained after the Tunguska event — only a patch of scorched earth 30 miles in diameter. So scientists have a paltry amount of evidence to investigate. The exact details may never be known.

Further reading:
What Caused the Tunguska Blast? 100 Years Later, Science Still Seeks Answers