The largest impact event in recorded history left no hole in the ground. That single fact, more than anything else, is why the explosion over the Tunguska river in 1908 has attracted a century of speculation. The science is now largely settled. The speculation has not gone away.
What happened
Shortly after seven in the morning of 30 June 1908, people across a wide stretch of central Siberia saw a column of bluish light move across the sky, followed by a flash and a series of bangs like artillery. At the Vanavara trading post, roughly 65 kilometres from the centre of the blast, a farmer named Semyon Semyonov later described the sky splitting open, a wave of heat strong enough to feel as if his shirt were on fire, and a shock that threw him off his porch. Evenki reindeer herders camped closer in lost animals, tents and stores; a handful of people are believed to have died. The pressure wave was recorded by barographs as far away as Washington and Batavia, and seismographs in Irkutsk, Tashkent, Tiflis and Jena registered a tremor of roughly magnitude five. For several nights afterward the sky over Europe glowed so brightly that Londoners reported reading newspapers outdoors at midnight.
The region was remote, the Russian state had other concerns, and nobody with scientific training went to look for almost twenty years.
The investigation
Leonid Kulik, a mineralogist with the Soviet Academy of Sciences, became convinced the reports described a giant meteorite fall. A 1921 journey got him as far as the railway towns, where he collected eyewitness accounts but could not reach the site. In 1927 he finally made it, by horse and on foot, to the Podkamennaya Tunguska basin. What he found was a forest laid flat in a radial pattern, trunks pointing away from a common centre, over an area later mapped at about 2,150 square kilometres. At the centre itself the trees were still standing, stripped of branches and scorched, like telegraph poles. Kulik searched for a crater through three more expeditions into the 1930s. He dug out bog holes he hoped were impact pits. He found nothing.
The absence of a crater was the puzzle, and the answer came from physics rather than digging. An object a few tens of metres across entering the atmosphere at high speed would be crushed by air pressure and detonate before reaching the ground. Modelling from the 1990s onward, including work by Mark Boslough at Sandia National Laboratories, showed that such an airburst between about five and ten kilometres up would produce exactly the pattern Kulik mapped: the standing trees beneath the blast, the radial fall outward, the scorching. Energy estimates range from about three to thirty megatons of TNT; NASA’s current description is of an asteroid roughly 40 metres across exploding about ten kilometres above the forest.
The explanations
Within the mainstream the open question is what the object was. Astronomers have long argued between a stony asteroid and a fragment of a comet, which would be mostly ice and dust and would vanish more completely. In 2013 a team led by Victor Kvasnytsya of the Ukrainian Academy of Sciences reported microscopic grains from a 1908 peat layer near the epicentre containing diamond, lonsdaleite and graphite with inclusions of troilite and taenite, minerals typical of meteorites, which they took as evidence of a stony or iron body. Not all researchers accept that the grains are securely linked to the event, and the debate continues.
In 2007 geologists from the University of Bologna proposed that Lake Cheko, a small, deep, funnel-shaped lake about eight kilometres from the epicentre, was gouged out by a surviving fragment. Later sediment studies, including a 2017 Russian analysis, indicated the lake is considerably older than 1908, and the idea has few supporters today. A separate line of work by Andrei Ol’khovatov has gone back to the Evenki accounts gathered in the 1920s and 1930s and argued that they fit a terrestrial geophysical event better than an impact; this remains a minority view.
Outside science the ideas are more colourful. In 1946 the Soviet writer Alexander Kazantsev published a story in which the explosion was a nuclear-powered alien spacecraft, and the notion took on a life of its own. Others have suggested a chunk of antimatter, a miniature black hole, a natural gas eruption, or an experiment by Nikola Tesla. None of these accounts for the evidence any better than an airburst, and several require physics for which there is no evidence at all.
Where it stands
Tunguska is real, and it is explained in outline. A body from space, most likely a stony asteroid, blew apart in the lower atmosphere and the shock wave did the rest. The 2013 Chelyabinsk fireball, a much smaller airburst that injured more than a thousand people, gave scientists a live demonstration of the same process. What remains uncertain is the composition and size of the object and whether any of it survived. The unexplained part of the story is small; the legend built on it is not.