Tunguska was a gas explosion.
http://www.thetimes.co.uk/article/0,,3-2001251300,00.html
Tunguska was one of the great mysteries of modern science. That may be about to change: 17 factors, including the patterns of tectonic faults and fallen trees in the area, suggest that the explosion had nothing to do with outer space, but was caused by gas forced upwards from the planet’s molten core, Wolfgang Kundt, Professor of Astrophysics at the University of Bonn, writes in August’s issue of the journal Current Science. A meteor 200ft across would have left at least 100,000 tonnes of debris along its approach path.