There is a theory that links in with tunguska, mass amounts of glass in the desert, and other events.
Scientists today believe that events such as tunguska, and other similar explosions in earths natural hitsories are caused by airbust events.
When a large meteroid or comet enters the earths atmosphere, they rarely hit the ground. The air compression caused in front of the object as it flies into the atmoshphere generates immense ammounts of heat, usually causing the object to burn up or explode. If it explodes near enough to the ground, and is of a decent size, it can cause immense explosions, many times more destructive than an actual impact. Tests have shown that these unique explosions cause the objects trail of fire to blossom out into a huge fireball in the upper atmoshpere. The explosions shock wave can knock down trees, and the heat from the explosion can melt sand into glass.
Similar events have happened all across the world, but Tunguska is the largest in human memory.
Far older events have been many times larger, with one in the desert in africa being an explanation for large amounts of glass (made of superheated sand) being scattered across an enormous area of the desert.
It is believed that there was once a massive, multiple airburst event in asia, which would have wiped out much of life in that part of asia.
Due to this theory, it is now believed that it is dangerous to try and destroy incoming comets/meteoroids that could be dangerous to civilisation, as fragmenting the object would make multiple airburst events be likely.