At about 7:17 in the morning on June 30, 1908, a light appeared in the sky over central Siberia. Witnesses near the Podkamennaya Tunguska River, a sparsely populated region of forest and bog, described a column of bluish-white fire moving across the heavens. Then came the heat, then the sound, then the wind.

The explosion flattened roughly 2,150 square kilometers of taiga, knocking down an estimated 80 million trees in a radial pattern that pointed outward from a central hypocenter. Seismographs as far as Britain registered the shock. Atmospheric pressure waves circled the globe twice.

For weeks afterward, observers across Europe and Asia reported night skies bright enough to read by. Newspapers in London printed photographs taken at midnight without flash. Nobody yet knew why.

The first scientific expedition did not reach the site until 1927, nearly two decades later. Leonid Kulik, a mineralogist at the Soviet Academy of Sciences, expected to find a meteorite crater. He found instead a vast zone of scorched and toppled timber with a curious feature at the center: a stand of trees still standing upright, though stripped of bark and branches. There was no crater. There were no meteorite fragments.

The absence of a crater launched a century of speculation. Kulik himself remained convinced a meteorite was responsible, hypothesizing that it had buried itself in swampy ground. Later expeditions searched for the impactor and found nothing.

On a June morning in 1908, something exploded over the taiga and flattened 80 million trees. For decades the cause was disputed. Recent modeling has narrowed the field considerably.

Other explanations followed. A small black hole passing through the Earth. A piece of antimatter annihilating in the atmosphere. The crash of an alien spacecraft, a theory popularized by the Soviet engineer Alexander Kazantsev in 1946 and rejected by essentially every scientist who looked at it.

The mainstream view by the late twentieth century had settled on an air burst — a body that exploded several kilometers above the ground rather than striking it. This explained the lack of a crater and the radial blast pattern. What remained disputed was whether the body was a stony asteroid or a fragment of a comet.

Recent modeling has tilted the answer toward asteroid. In 2020, a team led by Daniil Khrennikov at Siberian Federal University published simulations suggesting that the Tunguska object was an iron-rich asteroid roughly 200 meters across that entered the atmosphere at a shallow angle, grazed through it, and exited back into space. Under this model, the devastation came from the shock wave alone. The body itself never landed.

That hypothesis remains contested. Other teams favor a stony asteroid of perhaps 60 to 80 meters that fragmented and vaporized at an altitude of roughly 5 to 10 kilometers, depositing its energy as heat and pressure. Either model fits the surface evidence reasonably well.

What both rule out is the comet hypothesis. Comets are mostly ice and dust, and modeling suggests a cometary body of the required mass would have broken up too high in the atmosphere to flatten anything. The energy yield, estimated at 10 to 15 megatons of TNT equivalent, also fits an asteroid better than a comet.

Microscopic spherules of nickel and iron, consistent with asteroid material, have since been recovered from peat layers dated to 1908 in the affected region. Similar particles appear in tree resin collected from survivors of the blast. The evidence for an extraterrestrial origin is no longer in doubt. What kind of body, and what its precise trajectory was, remains a research question.

The human toll was remarkably small. The region was thinly inhabited by Evenki reindeer herders and a handful of trappers. Contemporary accounts describe several deaths, though Russian record-keeping in remote Siberia at the time was sparse. Had the same body arrived four hours later, the Earth's rotation would have placed Saint Petersburg, then home to roughly 1.5 million people, in the impact zone.

That near-miss is part of why Tunguska continues to attract attention. NASA's Planetary Defense Coordination Office cites the event as evidence that medium-sized impactors arrive more often than once thought — perhaps every few centuries rather than every few millennia. The agency now tracks roughly 35,000 near-Earth objects, with new ones added each week.

The Tunguska site itself has largely regrown. The trees that fell are still visible in places, preserved by the cold and wet of the taiga, but a younger forest has come up around them. Tourists occasionally make the journey, though it requires a helicopter or a long river trip.

What happened that morning in 1908 is now understood in broad outline, if not in every detail. A piece of the early solar system arrived without warning, did its damage, and either vaporized or returned to space. The forest is still recovering.