From an excerpt of Peter Brookesmith's book 'UFO: The Complete Sightings' (New York, 1995, pp 25):

"[On June 30, 1908,] Citizens of Vanavara [Siberia, Russia] saw and felt an enormous, blindingly bright, blue fireball streaking through the northern morning sky. Seconds later, a huge explosion came from the north-west, damaging buildings and knocking people flying. The object had detonated 40 miles away in the heavily wooded Stony Tunguska River valley, sending up a sinister mushroom-shaped cloud. The blast felled trees for up to 20 miles around and turned the forest into an inferno. The shock waves shook houses and knocked over animals as far as 400 miles away, and were powerful enough to circle the Earth twice... In the 1960's scientists estimated the force of the blast at between 2 and 30 megatons - that is, between 10 and 1500 times more powerful than the nuclear bomb dropped on Hiroshima. Amazingly, no human beings died as a result of the explosion."



100 years later, despite thorough investigations, the exact nature of this literally earth-shattering phenomenon remains a mystery. For more detailed information on the Tunguska Event:
en.wikipedia.org/wiki/Tunguska_event
https://www.psi.edu/projects/siberia/siberia.html