Just after seven in the morning on June 30, 1908, a farmer named Semyon Semyonov was sitting on the porch of a trading post at Vanavara, a small settlement in central Siberia. He had just raised his axe to fix a barrel hoop when, as he later wrote, the sky to the north split in two and seemed to fill with fire. The heat felt like his shirt was burning. Then came a bang and a crash that threw him about six meters off the porch and knocked him out for a moment. Windows in the trading post shattered. Vanavara was roughly 60 kilometers, or about 37 miles, from the center of whatever had just happened.
What happened was the largest impact event in recorded history, and it left no crater. Something from space exploded in the air above a remote stretch of forest near the Podkamennaya Tunguska River and flattened about 2,150 square kilometers of trees, an area of roughly 830 square miles. Scientists have spent more than a century, and by one estimate about 1,000 research papers, working out what it was. Here is what they found, what they still argue about, and why a blast over an empty forest in 1908 still shapes how we watch the sky today.
A second sun over the taiga
The region was thinly settled. Evenk families herded reindeer, hunted, and fished, living in conical tents called chums, and a few Russian villages sat along the rivers. Witnesses across central Siberia saw a fireball brighter than the Sun cross a clear sky from the southeast, trailing a thin white streak, before the explosion.
An Evenk man named Chuchan, whose family was camped much closer to the blast, described being woken by something pushing the tent, then a whistling sound and a strong wind. The tent was knocked flat. When he and his brother crawled out, trees were falling and their needles were burning, and he saw what looked like a second sun appear over the hills, followed by thunderclap after thunderclap. At least three Evenks are known to have died: one thrown against a tree whose broken arm became fatally infected, one who died of shock, and a shaman who likely died in one of the wildfires the blast set. Many Evenks were reluctant to talk about it afterward, partly because of customs around places where people had died, so the true toll is unknown.
Seismographs in Irkutsk, Tashkent, Tiflis, and Jena, Germany, picked up the shock, which was later estimated at close to magnitude 5. Barographs as far away as London, Batavia, and Washington, D.C., recorded the pressure wave, though nobody connected those readings to Siberia for decades. People across Europe noticed something stranger. For several nights after June 30, the sky stayed so bright that in some places you could read a newspaper outdoors around midnight. Astronomers writing in the journal Nature reported having to give up their night’s observations because the sky was too bright. Dust and ice thrown high into the atmosphere were catching sunlight from below the horizon, a cousin of the hazy skies that followed the eruption in our story on the Year Without a Summer.
Nineteen years to reach ground zero
Revolution, civil war, and sheer distance kept scientists away for years. The first expedition went out in 1921 under Leonid Kulik, a mineralogist surveying the region for the Soviet Academy of Sciences. His team never reached the center, but the local accounts convinced him that a giant meteorite had fallen. He persuaded the government to pay for a return trip, partly by pointing to the prospect of recovering valuable meteoric iron.
In 1927 Kulik hired Evenk hunters to guide him in. He expected a crater. Instead he found a zone roughly 8 kilometers across where trees were scorched and stripped of their branches but still standing upright. Beyond that zone, trees lay flat on the ground, their tops pointing away from the center in a vast radial pattern. Surveys in the 1960s showed that the downed forest had the outline of a giant butterfly, about 70 kilometers from wingtip to wingtip and 55 kilometers long.
Kulik went back several more times over the next decade, looking for the meteorite. He found dozens of small round bogs and suspected they were impact pits. His team spent a great deal of effort draining one of them, and at the bottom they found an old tree stump, which meant the bog was older than 1908. In 1938 he arranged an aerial photo survey of the central area. The 1,500 original negatives were later burned, in 1975, as part of an effort to get rid of flammable nitrate film, though positive prints survived. Kulik never found his iron. He died in 1942 as a prisoner of war after volunteering for a militia during World War II.
How do you flatten a forest without hitting it?
The answer is an air burst. Anything falling into Earth’s atmosphere from space arrives at no less than about 11 kilometers per second, more than 25,000 miles per hour. At that speed the air in front of the object can’t get out of the way. It piles up and compresses, heating the front of the rock and squeezing it with enormous pressure. Small meteoroids burn up as shooting stars. A larger rock can hold together deeper into the atmosphere, but eventually the pressure on its front face exceeds the strength holding it together. When that happens, it breaks into pieces, the pieces break apart even faster, and almost all of its energy of motion is released in a split second, many kilometers above the ground.
At Tunguska, the most widely cited models put the object at around 50 to 60 meters across, about the length of a 15-story building lying on its side, exploding somewhere between 5 and 10 kilometers up. Estimates of the energy vary a lot, from about 3 megatons of TNT to 30. A commonly cited figure of 10 to 15 megatons is roughly 1,000 times the first atomic bomb test. Some newer calculations argue that the falling object’s momentum pushed more of the blast downward than a nuclear weapon would, which means less total energy was needed to do the same damage.
The air burst also explains the odd pattern Kulik found. Directly under the explosion, the blast wave came straight down, snapping off branches but leaving trunks standing. Farther out, the wave arrived at a slant and pushed trees over like dominoes. In the 1960s, Soviet researchers built model forests out of matches stuck in a board and set off small charges above them on wires. They got the same butterfly shape, and from the experiments they worked out that the object came in at an angle of about 30 degrees from the ground.
Physical traces of the object are faint. Expeditions in the 1950s and 1960s sifted the soil and found microscopic spheres of silicate and magnetite with a high ratio of nickel to iron, a chemical signature typical of meteorites. Italian researchers in the 1990s found similar material trapped in tree resin from 1908. No large fragment has ever been confirmed.
Asteroid, comet, or something stranger?
For much of the twentieth century, the leading idea was a small comet. In 1930 the British meteorologist F. J. W. Whipple suggested that a body made mostly of ice and dust would vaporize completely and leave nothing behind, and the bright nights over Europe fit the idea of a dusty tail spread through the upper atmosphere. One version tied the object to Comet Encke, whose debris produces a meteor shower that peaks in late June.
The tide turned toward a stony asteroid. In 1983 the astronomer Zdeněk Sekanina argued that a fragile comet on such a shallow path should have broken up far higher in the sky. A 2001 study that modeled likely orbits put the odds at about 83 percent that the object came from the asteroid belt. Work after the 2013 Chelyabinsk meteor, which ran millions of combinations of size, speed, and strength, settled on a stony body of 50 to 80 meters as the best fit. Some researchers still argue for a comet or an iron asteroid, and one 2020 model even proposed an iron rock that skimmed through the atmosphere and flew back out into space.
Then there is Lake Cheko, a small lake about 8 kilometers from the epicenter. In 2007 a team from the University of Bologna proposed that it filled a crater dug by a fragment that survived the explosion, pointing to its cone-shaped floor and to sediment cores that seemed to show the lake forming around 1908. A 2017 Russian study counted at least 280 yearly layers of sediment in the lake bed, which would make it older than the blast. The question is still open.
A handful of researchers have proposed that the explosion came from below, from a sudden release of natural gas through an ancient volcanic vent under the site. That idea has little support, mainly because hundreds of witnesses described a fireball crossing the sky. And the popular story that Nikola Tesla caused the blast with a death ray has no basis at all. It traces back to a fictionalized biography published in 1994, and Tesla’s Wardenclyffe Tower was never finished.
Chelyabinsk: a smaller blast over a city
On the morning of February 15, 2013, people in the Russian city of Chelyabinsk got a preview of what Tunguska might have looked like over a populated area. An asteroid about 18 meters wide entered the atmosphere at roughly 19 kilometers per second and exploded about 30 kilometers up. For a moment it was brighter than the Sun. The blast released around 400 to 500 kilotons of energy, roughly 30 times the Hiroshima bomb.
Nobody saw it coming, partly because it approached from the direction of the Sun. And because the shock wave reached the ground minutes after the flash, many people had already gone to their windows to look at the bright streak when the glass blew in. According to official counts, 1,491 people sought medical care, mostly for cuts from broken glass, and around 7,200 buildings in six cities were damaged. No one died. Chelyabinsk also handed scientists a gift. Dashboard cameras, security cameras, and infrasound sensors recorded the event in detail, which gave researchers real measurements to test the models they had built for Tunguska.
How often this happens, and what we’re doing about it
Small air bursts are routine. Satellites and infrasound stations pick up explosions in the upper atmosphere that rival small nuclear weapons, and a blast of about 5 kilotons happens roughly once a year somewhere over the planet. Tunguska-scale events are much rarer. Older estimates put them at about once every 300 years, and more recent ones closer to once every thousand years. That’s rare enough that most of us will never see one, and common enough that space agencies treat the risk as real. Like the Carrington Event, Tunguska has become a benchmark for a natural hazard that is unlikely in any given year but very costly when it arrives.
The first line of defense is finding the rocks. Asteroids wider than 140 meters whose orbits cross Earth’s are classed as potentially hazardous, and surveys have been cataloging them for decades. The Vera C. Rubin Observatory in Chile began full survey operations in July 2026 and photographs the southern sky every few nights, and it is expected to multiply the number of known asteroids. NASA’s NEO Surveyor, an infrared space telescope built to spot dark asteroids that are hard to see in visible light, is scheduled to launch no earlier than September 2027.
The second line is moving them. On September 26, 2022, NASA deliberately crashed its DART spacecraft into Dimorphos, a 160-meter moonlet orbiting a larger asteroid called Didymos, at about 14,000 miles per hour. Neither rock threatens Earth. The goal was to see whether a head-on hit could change an asteroid’s path. NASA had set 73 seconds of change in Dimorphos’s orbit as the bar for success. The actual change was 32 minutes, largely because the debris blasted off the surface acted like a rocket exhaust and added to the push. The European Space Agency’s Hera spacecraft, launched in 2024, is due to arrive at the pair next month, in November 2026, to measure the crater and the asteroid’s mass up close.
Spacecraft deal with smaller space rocks every day. For the tiny, fast particles that pepper orbiting hardware, engineers use layered bumpers, which we covered in our look at the thin shields that stop space rocks going faster than bullets. Planetary defense is the same problem at a much larger scale, where the only practical shield is seeing the rock years ahead and nudging it early.
The forest today
The blast site is now part of the Tunguska Nature Reserve. New trees have grown up among the old fallen trunks, and satellite images like the one above show a forest that has largely recovered. Researchers still visit to take cores from peat bogs and trees, reading the 1908 layer the way Soviet drillers read deep rock at the Kola Superdeep Borehole. One speculative 2019 paper even proposed that the blast wiped out the breeding grounds of a little-known goose, which vanished in the following decades. The idea is unproven, but it shows how many questions the event still raises.
In 2016 the United Nations named June 30 International Asteroid Day, after the date of the blast. The lesson of Tunguska is simple. A rock no bigger than an office building, arriving without warning, flattened a forest the size of a major city. It happened to hit one of the emptiest places on Earth. The telescopes and test spacecraft of the past decade are about making sure that next time, we see it coming.
Hold a piece of a fireball, or dig deeper into the mystery
KALIFANO Sikhote-Alin Meteorite Bundle (3 Pieces) — Real iron meteorite fragments from the Sikhote-Alin fall, which rained down on mountains in Russia’s Far East in 1947, about 39 years after Tunguska. This set comes with an information card and a certificate of authenticity, so it makes a good desk piece or gift for a space fan.
The Mystery of the Tunguska Fireball — Surendra Verma’s short, readable book on the 1908 blast walks through the eyewitness accounts, the expeditions, and the long list of explanations people have proposed since, some serious and some wild.