June 30, 1908, shortly after 7am observers at the Central Siberian
Plateau near the Stony Tunuska River first saw a "ball of fire" coming
over the southern horizon heading almost due north.  Suddenly a
"pillar of fire" short upward into the sky, which could be seen for
several hundred miles.  Enourmous black clouds rose 12 miles into the
air, followed by a "black rain".  On the following day, strange
glowing clouds could be seen, at extremely high altitudes over Asia
and Europe.

   Various explainations for the explosion, which devastated several
hundred square miles of (furtunately unpopulated) pine forest have
been put forward.  

    * The original one, that of a meteor, was shot down due to there
being no physical meteoric evidence to be found.  

    * An alternative explaination was that of a comet entering the
atmosphere and then violently vaporizing due to the increased heat.
(The EoU calls this "the accepted, conservative view").  Since coments
haven't been observed striking the earth before (or since) this event,
it remains a possibility.

    * In 1947, the "spaceship theory" was put forward, that a
spaceship's powerplant detonation had caused the devastation.
(Or, alternatively, that the aliens detonated a nuclear device
on purpose, for reasons of their own - I've even heard tell of
"the war of the aliens", and such.)

    * Although not mentioned in EoU, another theory surfaced in the
mid-1970's, that of a "mini-blackhole" (described by someone else in
an earlier message), that actually passed through the earth.
Unfortunately, no effects on the far side of the earth (waterspouts,
or whatever) were witnessed (to my knowledge), so there's no extra
evidence for this one.

   The EoU goes on in some detail, I'll just hit the high points:

 Points against the "spaceship" theory:
	* The fireball was far too brief, in comparison with what we
	  know now about nuclear explosions, to be one.
	* approach trajectory of the object has been determined
	  to be head-on with earth, which isn't really a great
	  idea to do with a spaceship, so it seems unlikely that
	  any self-respecting alien would do so.
	* there is no increase in carbon-14 in tree-rings that would
	  account for a nuclear explosion in 1908 (this was asserted
	  by William Libby, the 'father of the carbon-14 test', in 
	  1976).
	* The recollections of witnesses was 20 years old before
	  it was obtained by researchers.
	* spectrographic analysis (by astronomers) in 1908 showed
	  that the "glowing cloud" was just dust, reflecting sunlight.
	* There had been a forest fire in 1888 in the same region, 
	  which accounts for descriptions of "rapid growth" in plants
	  after the explosion; rather than being radiation-related,
	  it was just due to nutrient-enhancement that happens to
	  soil after a forest fire (and that the regions of
	  "rapid growth" were in the areas of forest-fire, and not
	  in other areas.)

... there's more, discussing further problems with "meteor" and
"comet" theories, but this is a.a.v, and besides, you should find
this book for yourself - try the library...

   I'll end with last line from the entry: "Whatever the actual 
cause of the Tunguska event, it has maintained a powerful grip
on the popular imagination."

Hope this helps,

Charles