from a meteoroid air burst. 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 carbon, hydrogen, and nitrogen 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 Cretaceous–Paleogene boundary. These unusual proportions are believed to result from debris from the falling body that deposited in the bogs. The nitrogen is believed to have been deposited as acid rain, a suspected fallout from the explosion. However other scientists disagree: "Some papers report that hydrogen, carbon and nitrogen isotopic compositions with signatures similar to those of CI and CM carbonaceous chondrites were found in Tunguska peat layers dating from the TE (Kolesnikov et al. 1999, 2003) and that iridium anomalies were also observed (Hou et al. 1998, 2004). Measurements performed in other laboratories have not confirmed these results (Rocchia et al. 1990; Tositti et al. 2006).". Researcher John Anfinogenov has suggested that a boulder found at the event site, known as John's stone, is a remnant of the meteorite, but oxygen isotope analysis of the quartzite suggests that it is of hydrothermal origin, and probably related to Permian-Triassic Siberian Traps magmatism. In 2013, a team of researchers published the results of an analysis of micro-samples from a peat bog near the centre of the affected area, which show fragments that may be of extraterrestrial origin. Earth impactor model The leading scientific explanation for the explosion is a meteor air burst by an asteroid above the Earth's surface. Meteoroids enter Earth's atmosphere from outer space every day, travelling at a speed of at least . The heat generated by compression of air in front of the body (ram pressure) as it travels through the atmosphere is immense and most meteoroids burn up or explode before they reach the ground. Early estimates of the energy of the Tunguska air burst ranged from to 30 megatons of TNT (130 PJ), depending on the exact