In February, 1996 ZetaTalk stated that Tunguska was due to methane released during an earthquake; articles and discussion with Russian scientists in March 1997 showed the Tectonic Explanation more accurate, backed by Iridium Concentration and Radiocarbon data, and Andrei Yu Ol'khovatov and the BBC argue that with Methane Evidence. On July 21, 2001 The Times of London reported that German scientists had concluded a Gas Explosion, and by Sep 2, 2002 the Russians Agreed.
July 10, 1998
A couple days ago I have returned from Tunguska (the conference and the epicenter). A success of 'endogenists' was larger than I anticipated. Despite we were a minority, we advanced, and 'meteoritists' retreated a little. Anyway, finishing the conference, the informal leader of the Tunguska research - Nikolai Vasilyev had to say that unexpectedly an idea that the Tunguska was terrestrial origin is coming back, based on another idea. I think since that time 'meteoritists' can't ignore it anymore. But, of course, it is just a beginning. Possibly one more 'dark cloud' for 'meteoritists' is appearing (possible finding of another large forest dated probably 1908 with very high iridium concentration). If future research will prove it, it will be a devastating strike on 'meteorite Tunguska'.
Yonenobu, H. & Takenaka C. The Tunguska event as recorded in a tree trunk. Radiocarbon 40: 367-371 (1998). Basically, they present isotopic evidence that the event was not due to a comet.
In CCNet April 10, 2001, Matthew Genge posted his opinion on presense of extraterrestrial substance in the Tunguska epicenter. Here I'd like to make a couple of remarks.
The isotopic ratio for Tunguska epicenter's REE measured in the place of the most REE enrichment is the terrestrial one. Regarding C-14, its data is rather contraversial, but the latest measurements seem to show no significant deviations against background. And is it plausible to propose that over the epicenter the extraterrestrial substance was dispersed into fine microparticles during the powerful Tunguska explosion and flew away, explaining why there was no fall out? The latest published calculations of the hypothetic Tunguska spacebody's explosion show that a plasma column of the remnants was to strike the ground. So this scenario (99.999...% of the substance flew accurately away from the epicenter!...) looks very unlikely.
But the problem is even much more deep. Where is enormous quantity of extraterrestial substance, which the hypothetic Tunguska body was to loose before its explosion? Indeed, according to practically all calculations, the hypothetic Tunguska spacebody lost a significant (probably even dominated) portion of its mass before it reached the explosion altitude of 5-8 km. In other words, at least, a hundred thousands tonns of extraterrestrial substance (from large fragments to dust) must be deposited along the lower part of Tunguska spacebody trajectory. Despite many years of careful search nothing was discovered... By the way, a couple years ago application of just one of the many Tunguska substance search's methods allowed easily find small fragments of a meteorite (with initial mass of several tonns, probably), which is integrated over another place in Siberia in 1904...
But the substance problem is just one of many other unresolved Tunguska mysteries. Here I can just repeat words of recently deceased leader of Tunguska research, academician Vasil'ev that we don't know what Tunguska was....