Here is a note from SCIENCE magazine - a reaction on the above-mentioned discussion of Tunguska at the Brunel Univ. conference (SCIENCE's issue of Sept.13, 2002, p.1803):

MORE THEORIES ON TUNGUSKA

On the morning of 30 June, 1908, in the remote Tunguska forest of Siberia, a vast explosion charred and flattened trees across an area nearly as large as Rhode Island.
Scientists have been mystified for years over the cause, although prevailing wisdom has it that an extra-terrestrial chunk of ice or rock struck Tunguska (Science, 20 August 1999, p. 1205).
At a conference in London last week on environmental catastrophes, physicists from Russia and Germany rejected the 'ET hypothesis.' Andrei Ol'khovatov, formerly of the Soviet Radio Instrument Industry Research Institute, noted that decades of research has failed to find definitive traces of extraterrestrial material. No impact crater has been found and, more tellingly, patches of trees were left untouched near the epicenter of the blast.
Wolfgang Kundt, an astrophysicist from Bonn University, Germany, then proposed an alternative scenario: a massive gas explosion. A large natural gas deposit lies below the site, a well-known fact unconnected to the event until now, he said. Kundt has modeled a Tunguska 'outgassing' and says it would fit with eyewitness accounts of the event. "The geophysical hypothesis is a fresh idea," says Jesus Martinez-Frias of the National Aerospace Institute in Madrid, Spain "It could be the answer." But Ol'khovatov believes Tunguska was a "poorly understood strong coupling between subterranean and meteorological phenomena" that science is not yet ready to understand.