At about 7:17 on the morning of Tuesday, 30 June 1908, in a remote corner of central Siberia roughly a thousand kilometers north of Lake Baikal, something the size of an apartment building entered the atmosphere at twenty-seven kilometers per second, compressed the air in front of it to the temperature of the surface of the Sun, and detonated about eight kilometers above the ground with a force of approximately twelve megatons of TNT.
The blast wave flattened, in the moments that followed, approximately 2,150 square kilometers of taiga forest — every tree within a radius of about thirty kilometers from the airburst’s ground zero, knocked over and laid pointing radially outward like grass after a heavy boot. The wave was strong enough to set fires across the central area, knock a man named Semen Semenov off his porch at a trading post sixty-five kilometers away, and register on seismographs in Russia, in Germany, in the United Kingdom, and in the United States. The atmospheric pressure wave circled the Earth twice.
There was no crater. There was no recovered meteorite. There were almost no human witnesses, because the airburst happened over one of the least populated regions of the inhabited world, and the few witnesses who existed were almost all illiterate hunters and herders whose statements were not collected for years.
The largest natural explosion of the twentieth century took place in front of perhaps two hundred direct observers and was not seriously investigated by anyone until nineteen years later.
What Semenov saw
The clearest contemporary account comes from Semen Borisovich Semenov, a Russian farmer and trader stationed at the trading post of Vanavara, on the Podkamennaya Tunguska River. Semenov was sitting on the front porch of the post’s main building at the moment the blast wave arrived. He gave his testimony to the Russian-Soviet meteorite specialist Leonid Kulik on the latter’s 1927 expedition, nineteen years after the event. His description, recorded by Kulik:
“Suddenly in the north sky the sky was split in two, and high above the forest the whole northern part of the sky appeared covered with fire. At that moment I became so hot that I couldn’t bear it, as if my shirt was on fire; from the northern side, where the fire was, came strong heat. I wanted to tear off my shirt and throw it down, but then the sky shut closed, and a strong thump sounded, and I was thrown a few meters.”
Semenov was sixty-five kilometers from the epicenter. The thermal pulse he felt was an instantaneous spike in radiated heat from the fireball — the same physical effect that, in the much larger nuclear airbursts of 1945, would burn the shadows of bodies into pavement at comparable distances. He survived because the duration of the pulse was short and his clothing did not ignite.
Other eyewitnesses across central Siberia reported similar things. A column of bluish-white light, brighter than the sun, climbing into the sky from the north. A sound like artillery fire that continued for several minutes. A blast of hot air that knocked over horses. A geomagnetic disturbance was recorded at the Irkutsk observatory. The atmospheric scientist Andrei Voznesensky, then the director of the Irkutsk Magnetic Observatory, collected eyewitness statements from sixty Siberian villages over the following weeks and forwarded them to the Imperial Russian Geographical Society in Saint Petersburg.
The Geographical Society filed the report, found it interesting, and did nothing with it. Russia was preparing for revolutionary upheaval. There was no expedition. There was no follow-up. The site was 800 kilometers from the nearest railway and four weeks’ march through swampy taiga in the summer or freezing tundra in the winter. No scientist who heard about Tunguska in 1908 believed any of them would ever go there.
What Kulik found
Leonid Alekseevich Kulik was the chief specialist on meteorites at the USSR Academy of Sciences in Leningrad. He had heard about Tunguska in the early 1920s, looked into the archived Geographical Society reports from 1908, and concluded that the event had probably been a large iron meteorite impact. If so, he reasoned, there had to be a crater, and the crater had to be full of recoverable iron. He spent four years persuading the Soviet government to fund an expedition.
The first expedition reached the area in spring 1927. Kulik’s team — three scientists, two Evenki guides, and a series of pack horses — traveled by river and on foot from Vanavara into the central blast zone. They expected to find a crater. They found, instead, a fan of flattened trees radiating outward across hundreds of square kilometers, with a central area of standing dead trees stripped of branches, indicating an airburst rather than a ground impact. Kulik had been wrong about the iron meteorite. He had also been wrong about being able to find it.
He searched for the better part of the summer. He dug into bogs that looked like they might have been craters. He drilled into the permafrost in the central zone. He recovered no extraterrestrial material. He returned to Leningrad with photographs, eyewitness interviews, and a hypothesis — that the body had been a comet or a friable stone, vaporized completely in the airburst — that he could not prove.
He went back in 1928 and again in 1929 and again in 1938. He never found a meteorite. He continued to believe one was there, somewhere, and that his expedition methods were inadequate.
Kulik was conscripted into the Soviet army in 1941 in his late fifties, was captured by the Germans, and died of typhus in a prisoner-of-war camp in occupied Poland in April 1942. His personal papers, including notebook drafts of a never-finished monograph on Tunguska, are preserved at the Russian Academy of Sciences archive in Moscow. They have been only partially translated into English.
What it actually was
Modern reconstruction, based on the 1908 eyewitness data, the seismographic record, the pattern of flattened trees, and laboratory simulations of high-altitude atmospheric disruption, is that the Tunguska body was a stony asteroid roughly fifty to eighty meters in diameter that entered the atmosphere at a shallow angle and exploded at an altitude of about five to ten kilometers due to the aerodynamic stresses of its high-velocity passage. The 1993 paper in Nature by Chyba, Thomas, and Zahnle gave the first quantitatively rigorous model of why a stony asteroid of this size would airburst rather than crater. The model has held up well.
The released energy of twelve megatons is roughly a thousand times the yield of the Hiroshima bomb and equivalent to a modest hydrogen weapon. If the same body had entered the atmosphere a few hours later, the Earth’s rotation would have positioned the airburst over Saint Petersburg. The city would have been destroyed. The Russian Revolution, in some plausible counterfactual, would not have happened — because Lenin would have died of blast injuries in his exile flat in Geneva nine years before he could return.
This was the impact event astronomers had to take seriously after 1993. The realization that an object too small to be tracked by ground-based telescopes could destroy a city on arrival, with no warning, drove the modern programs for cataloguing near-Earth asteroids. The PanSTARRS, Catalina, and ATLAS surveys exist, in significant part, because of Tunguska. The 2013 Chelyabinsk meteor — a stony asteroid about twenty meters across that airburst over an inhabited Russian city, injuring 1,500 people, also undetected before arrival — was the reminder that the surveys are not yet adequate.
What’s still in the ground
In 2007 an Italian-Russian expedition led by Luca Gasperini at the University of Bologna proposed that a small lake near the Tunguska blast zone, Lake Cheko, was actually the impact crater of a fragment of the airburst that had reached the ground. The lake is roughly circular, about 500 meters across, with a profile compatible with a small impact. The seabed sediment dates to within a few decades of 1908. The hypothesis has been disputed by Russian geologists, who argue the lake is a normal taiga lake of older age. The question remains unsettled.
The blast zone itself has long since regrown. Aerial photographs from the 1960s show the patchy regrowth pattern; modern satellite images show full forest cover, mostly fir and birch, the older flattened trunks still visible underneath as a layer of darker decaying wood. A small protected zone, the Tungussky State Nature Reserve, was established by the Russian government in 1995 around the central area of the airburst. There is a single hut, used by visiting scientists and the occasional adventure tourist. The hut is the only structure within fifty kilometers.
Kulik’s original camp from 1927 is still there. The fire pit is identifiable. A few rusted equipment fragments remain.
The body itself, whatever it had been, evaporated in the upper atmosphere on the morning of 30 June 1908 and dispersed as particulate dust across the troposphere over the following weeks. The night skies across Europe were unusually bright for several months afterward. People in London read newspapers on the street at midnight. They had no idea why.