Tunguska is a place in Siberia, Russia where the Tunguska Event took place. In 1908, a mysterious event occurred that flattened 50 kilometers of forest with no additional evidence. Theories suggest that an asteroid or part of a comet was responsible, but Mulder believed the cause to be more alien in nature.
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The first recorded expedition arrived at the scene more than a decade after the event. In 1921, the Russian mineralogist Leonid Kulik, visiting the Podkamennaya Tunguska River basin as part of a survey for the Soviet Academy of Sciences, deduced from local accounts that the explosion had been caused by a giant meteorite impact. He persuaded the Soviet government to fund an expedition to the Tunguska region, based on the prospect of meteoric iron that could be salvaged to aid Soviet industry. Kulik's party eventually undertook an expedition in 1927.
Upon arrival, Kulik made arrangements with the local Evenki hunters to guide his party to the impact site. Reaching the explosion site was an extremely arduous task. Upon reaching an area just south of the site, the superstitious Evenki hunters would go no further, fearing what they called the Valleymen. Kulik had to return to the nearby village, and his party was delayed for several days while they sought new guides.
The spectacle that confronted Kulik as he stood on a ridge overlooking the devastated area was overwhelming. To the explorers' surprise, no crater was to be found. There was instead around ground zero a vast zone (8 kilometers [5 mi] across) of trees scorched and devoid of branches, but standing upright. Those farther away had been partly scorched and knocked down in a direction away from the centre. Much later, in the 1960s, it was established that the zone of leveled forest occupied an area of some 2,150 square kilometres (830 sq mi), its shape resembling a gigantic spread-eagled butterfly with a “wingspan” of 70 kilometres (43 mi) and a “body length” of 55 kilometres (34 mi). Upon closer examination, Kulik located holes which he erroneously concluded were meteorite holes; however, he did not have the means at this time to excavate the holes.
During the next ten years there were three more expeditions to the area. Kulik found several dozens of little “pothole” bogs, each some 10 to 50 metres (33 to 160 ft) in diameter, that he thought might be meteoric craters. After a laborious exercise in draining one of these bogs (the so-called “Suslov’s crater”, 32 metres [105 ft] in diameter), he found there was an old stump on the bottom, ruling out the possibility that it was a meteoric crater. In 1938, Kulik arranged for an aerial photographic survey of the area  covering the central part of the leveled forest (some 250 square kilometres [97 sq mi]). The negatives of these aerial photographs (1,500 negatives, each 18 ×18 cm or 7.1 x 7.1 in) were burned in 1975 by order of Yevgeny Krinov, then Chairman of the Committee on Meteorites of the USSR Academy of Sciences. It was done under the pretext that they were a fire hazard, but the truth may have been the active dislike by official meteorite specialists of anything associated with an unyielding enigma. However, positive imprints could be preserved for further studies in the Russian city of Tomsk.
Despite the large amount of devastation, there was no crater to be seen.
Expeditions sent to the area in the 1950s and 1960s found microscopic silicate and magnetite spheres in siftings of the soil. Similar spheres were predicted to exist in the felled trees, although they could not be detected by contemporary means. Later expeditions did identify such spheres in the resin of the trees.Chemical analysis showed that the spheres contained high proportions of nickel relative to iron, which is also found in meteorites, leading to the conclusion they were of extraterrestrial origin. The concentration of the spheres in different regions of the soil was also found to be consistent with the expected distribution of debris from a meteorite airburst. Later studies of the spheres found unusual ratios of numerous other metals relative to the surrounding environment, which was taken as further evidence of their extraterrestrial origin.
Chemical analysis of peat bogs from the area also revealed numerous anomalies considered consistent with an impact event. The isotopic signatures of stable carbon, hydrogen, and nitrogen isotopes at the layer of the bogs corresponding to 1908 were found to be inconsistent with the isotopic ratios measured in the adjacent layers, and this abnormality was not found in bogs located outside the area. The region of the bogs showing these anomalous signatures also contains an unusually high proportion of iridium, similar to the iridium layer found in the K–T boundary. These unusual proportions are believed to result from debris from the impacting body that deposited in the bogs. The nitrogen is believed to have been deposited as acid rain, a suspected fallout from the explosion.