On the morning of June 30, 1908, in a remote area of eastern Siberia, a strange sight was witnessed in the sky. It began as a column of blue light moving through the atmosphere over the Podkamennaya Tunguska River. Minutes later, a flash filled the horizon and a deafening boom shook the earth. This phenomenon, known as the Tunguska Event, would become known as the largest impact event in recorded history. Shockwaves from the blast could be measured from across the globe. An estimated 80 million trees were flattened, and for nights after the skies over Eurasia could be seen glowing. As massive as the impact was, much remains mysterious about the Tunguska Event, even down to what exactly caused it.

The Podkamennaya Tunguska River runs for a thousand miles, deep within the Russian Taiga. This region is difficult to reach, but for over a century scientists have trekked to study and retrieve physical evidence of the event. In 1921, the first team of scientists headed by mineralogist Leonid Kulik began investigating the area. Their first expedition did not lead them to the impact site, though they did conclude that the event was due to a meteorite. On subsequent trips, Kulik’s team found the center of the destruction, a 5-mile patch of forest full of scorched trees stripped of their branches. Strangely, there was no impact site found.

Further investigations of the site showed that while there was no crater, there was evidence of an extraterrestrial body. Microscopic spheres of silicates and magnetite could be found in the soil and lodged in some trees. This metal had a high nickel-to-iron ratio, a quality commonly found in meteorites. The tiny size of the residue and lack of a crater suggests that the object exploded in midair and disintegrated before it hit the ground. Because of this, little is known about the actual object behind the Tunguska Event.

There is speculation that the object may have been either a meteorite or a stony comet somewhere around 200 feet (60 m) in size. The altitude of the explosion is uncertain, though it must have been at least 3 miles. Even the strength of the blast is unknown, with estimates ranging from 3 to 30 megatons of TNT, over a thousand times stronger than the Hiroshima blast. What is known is the area affected by the event. It disrupted over 830 square miles (2,150 sq km) of forest and left behind a significant amount of suspended particles in the atmosphere. A blast of that size could have easily destroyed a metropolitan area.

Even over a century later, scientists continue to reconstruct the event to try and determine more about the impactor. The lack of any large asteroid fragments supports the comet hypothesis, along with the fact that cometary material is more brittle and would be more likely to fragment completely in the atmosphere at around 10km. Eye witnesses reported afterwards that the object moved at 28 degrees or less; coupled with the trail of flattened trees, one can reconstruct the path and angle of the impact. Curiously, the trees along the flight path heading to the impact site do not show strong shockwaves, another point for the brittle comet hypothesis.

Being in a remote area, property damage and human casualties were minimal, but the region did not go unscathed. Injuries from such an event can include thermal radiation from the high heat, concussion, and lung damage from shockwaves, environmental contamination from outside elements, not to mention objects being hurled as projectiles. Few stories of witnesses have been recorded, but a 1926 Russian ethnographic mission recorded two testimonials of severe injuries directly related to the impact, including a man being thrown against a tree, leading to infection and death. These reports come from the Evenki nomadic people who inhabit the region.

The sudden and destructive nature of the Tunguska Event gives one pause about the power of future meteoric events. History shows possible records of massive meteorites lighting up the sky in ancient Greek, South American, and Babylonian stories. A Tunguska-class impact is estimated to be a thousand-year event, meaning the likelihood of such an impact occurring again is relatively low. That said, in 2013 an airburst of about half the power of Tunguska occurred over Chelyabinsk, Russia, causing considerable damage and raising the possibility of another destructive impact sometime in our planet’s future.