One hundred years ago this month, an explosion occurred over the Tunguska river area of Russia that was hundreds of times stronger than the atomic bomb that was dropped on Hiroshima.  It knocked down about 80 million trees.  There have been a number of theories formed about what caused it, but so far there is no conclusive evidence to back up any one of them.  
    The more scientifically oriented tend to believe it was either an asteroid or a comet.  Since no fragments of an asteroid have been found there, not many accept that idea.  The comet Idea makes more sense because a comet made up largely of ice would break apart and melt.  Other alternative ideas include a flying saucer exploding, a collision with a small black hole or an anti-matter meteorite.  None of these theories have any real proof either.  
The most accepted theory now is that it was a meteorite that burst in the air about ten kilometers above the ground.
    The scary part is that as far as we know, whatever caused it could happen again and this time it might not be over unpopulated forests.