The theories put forth to account for the Tunguska event range from the scientifically plausible to the ridiculous to the intriguing. They included:
A fragmentary asteroid or meteorite that exploded in the atmosphere.
The nucleus of a comet that likewise exploded in the atmosphere.
An unusual tectonic event.
A tiny black hole that entered the Earth's atmosphere from outer space and imploded. A chunk of antimatter that reacted with the matter of our planet.
A crashed UFO, the propulsion drive of which exploded.
A deliberate attack by extraterrestrials.
The result of a test of Nikola Tesla's wireless power transmitter.
Again, there's no definitive proof for any of these ideas, but let's consider each.
Asteroid - This and the comet theory are favored by scientists, of course - mainly because they can't conceive of any other explanation.
I'd have to agree that it's the most likely. But because there is no crater and little debris, there's only circumstantial evidence. Before Tunguska, scientists rarely considered that an asteroid would explode in the atmosphere before striking the ground. Yet, because there is no crater, they reason, that must be what happened. So where are all the fragments of the asteroid that they estimate must have weighed some 100,000 tons? Vaporized, they say - pulverized into dust and tiny gravel. The only fragments found thus far have been tiny glass nodules embedded in the fallen trees, which are consistent in makeup with stony asteroid fragments that have been super-heated.
Comet - This is the prevailing theory today - that it was a 100,000-ton fragment of Encke's Comet. Since there is little debris, the explosion might be consistent with a comet, which generally is a loose mixture of stone and ice. Upon explosion, very little debris would remain as evidence. Ironically, it is the very lack of evidence that boosts the credibility of the comet theory.
An unusual tectonic event - Andrei Yu. Ol'khovatov, a Russian scientist, has recently come up with the interesting, plausible theory that Tunguska was "a geophysical event, associated with tectonic processes" - a powerful earthquake, the enormous pressures of which also resulted in the recorded atmospheric effects.
Black hole - This idea isn't taken very seriously by mainstream scientists, simply because it's not known whether such small black holes even exist. And if they did, what the result would be upon one entering our atmosphere is completely unknown.
Antimatter - This idea is also readily dismissed, since it is unlikely that antimatter would be able to transverse space and reach our planet without already encountering some matter and annihilating.
Crashed UFO - There's no evidence whatever of this idea, of course. No fragments of the spacecraft or piece of an alien's intergalactic map. If it were the explosion of the UFO's propulsion system - nuclear or whatever - it might have vaporized all traces of the ship, but come on....
Extraterrestrial attack - If they were going to attack, why would they choose an unpopulated region, unless their intelligence was bad? Or unless it was meant as just a warning. And it it was just a warning, where was the follow-up or contact?
Nikola Tesla's experiment - Granted, this idea is far more unlikely than an asteroid or comet strike, but I find it quite a bit more interesting. A lot of myth has grown around the mysterious, dark and temperamental figure of Tesla. Although known as the discoverer of the principals of alternating current and other inventions, he is also credited in some quarters with far more notorious inventions, including a death ray. Some say the controversial HAARP array in Alaska is a continuation of Tesla's experiments that used electricity to create super weapons. The Tunguska event, they say, was the result of a test of such a weapon - a test that didn't go exactly as planned.
