Trees knocked over by the Tunguska blast. Photograph from Kulik's 1927 expeditionToday marks the 100th anniversary of the Tunguska Event, a rather large explosion in Siberia, for which there is still not a definitive explanation (though most scientists favor either the meteorite or comet theory).

The blast, estimated as somewhere between 5 and 30 megatons, knocked down tens of millions of trees over hundreds of square miles. Hard to imagine, and the pictures don't quite capture the scale of such devastation.

One of the more interesting aspect is the fact that there were trees at the center of the blast radius still standing, though stripped of branches and bark. The theory goes that the blast occurred several miles up, and the force of the blast heading straight down stripped the trees, but left them standing.

So later this week, if you're in the U.S. and singing the National Anthem on Independence Day, give a little thought to "the bombs bursting in air" that are completely natural.

The universe is a pretty awe-inspiring place...and not always safe. But I wouldn't want to live anywhere else.