During the next 10 years, there were three more expeditions to the area. Kulik found several dozen little "pothole" bogs, each 10 to 50 metres (33 to 164 feet) 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 m [105 ft] in diameter), he found an old tree 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 (250 square kilometres [97 sq mi]). The original negatives of these aerial photographs (1,500 negatives, each 18 by 18 centimetres [7.1 by 7.1 inches]) were burned in 1975 by order of Yevgeny Krinov, then Chairman of the Committee on Meteorites of the USSR Academy of Sciences, as part of an initiative to dispose of flammable nitrate film. Positive prints were preserved for further study in Tomsk.