On the evening of June 30, 1908, the skies over Eastern Europe glowed an eerie green. Earlier that day, witnesses throughout parts of Russia had observed an intensely bright, bluish white column of light appear in the sky, which after ten minutes seemingly collided with the earth, producing widespread destruction throughout the Siberian Taiga near the remote area of the Stony Tunguska River.
Believed to have been a comet or meteor that struck the earth, no impact of similar magnitude had ever occurred in recorded history, and the Tunguska event of 1908 remains the largest potential impact event in modern times.
Descriptions of the event, given around the time of the blast, often read like the following account, which appeared in the Sibir newspaper on July 2nd, 1908:
"[A]round 9 a.m. in the morning, we observed an unusual natural occurrence. In the north Karelinski village the peasants saw to the north west, rather high above the horizon, some strangely bright (impossible to look at) bluish-white heavenly body, which for 10 minutes moved downwards. The body appeared as a "pipe", i.e., a cylinder. The sky was cloudless, only a small dark cloud was observed in the general direction of the bright body. It was hot and dry. As the body neared the ground, the bright body seemed to smudge, and then turned into a giant billow of black smoke, and a loud knocking (not thunder) was heard, as if large stones were falling, or artillery was fired. All buildings shook. At the same time the cloud began emitting flames of uncertain shapes. All villagers were stricken with panic and took to the streets, women cried, thinking it was the end of the world."
It is easy to understand why observers would have likened this blast to the "end of the world," though despite its magnitude, much of the cause behind the seemingly cosmic incident still remains mysterious. Among the most perplexing issues raised by subsequent investigations into the cause of the Tunguska event had been the lack of any discernible crater for the impact, which seemingly points to an airburst, in which the object broke up in midair, rather than colliding with Earth.
There are less popular alternative theories that have been espoused over the years, as well. One of these, proposed by the astrophysicist Wolfgang Kundt of the University of Bonn, who suggested that rather than an large meteor or comet impact, the incident could have resulted from geophysical phenomenon instead... namely that of a massive buildup of natural gas.
Unlikely though it may seem with regard to descriptions of the aerial phenomena that preceded the blast, Kundt's hypothesis suggests that somewhere in the neighborhood of 10 million tons of natural gas could have leaked from the Siberian taiga, rising high into the atmosphere above Tunguska (conceivably, if this were indeed the case, it might account for some of the luminous phenomenon witnesses described). Following the appearance of this massive, gaseous plume, an ignition source (Kundt and others have proposed lightning as one possible source) could have set the gas plume ablaze, which would have created a column of fire which burned from its ignition point all the way back to the terrestrial source, where the explosion occurred.
In his paper "The 1908 Tunguska catastrophe: An alternative explanation", Kundt presented more than seventeen reasons the Tunguska blast may not have been an impact with an extraterrestrial object, "but rather by the volcanic ejection of some 10 Mt of natural gas." Kundt argues in the paper's abstract that, "The Tunguska event may well have been the present-day formation of a kimberlite," referencing an explosive outflow of vented natural gas and its "buoyant escape towards the exosphere."
There have been similar interpretations of the Tunguska blast offered within the scientific community over the years, which includes a theory outlined in the very interesting paper, "Geophysical Circumstances of the 1908 Tunguska Event in Siberia, Russia", by A. YU. Ol'Khovatov. In it, the author argues that, "research reveals that the event took place during a strong upsurge of tectonic activity in the Tunguska event region, and there were some peculiarities in tectonic activity even on larger scales. Also the event occurred during a change from a long period of "good" weather to a “bad” one in the region. And there were also peculiarities in the atmosphere on larger scales at those times.
"In the author’s opinion,"Ol'Khovatov writes, "this suggests that the Tunguska event was of geophysical origin. On much smaller scales similar geophysical events occur rather often."
Despite evidence that suggests a geophysical origin of the Tunguska blast, the meteorite of comet impact theory remains the most widely accepted cause. However, it may be that with time, and the acquisition of new data, a clearer understanding of the true cause of the Tunguska event will come to light, and with it, the potential for warding off any similar future events; whether they be terrestrial, or extraterrestrial in origin.