On 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 long been mystified as to the cause, although prevailing wisdom has it that it was an extraterrestrial chunk of ice or rock (Science, 20 August 1999, p. 1205). Tunguska epicenter today. CREDIT: VITALII ROMEIKO But two scientists last week rejected the "E.T. hypothesis" at a conference on environmental catastrophes in London. Andrei Ol'khovatov, formerly of the Soviet Radio Instrument Industry Research Institute, noted that no one has ever found definitive traces...