Russia was in a bad way in 1908. The Russian empire, already on its last legs, had been humiliated by Japan in a naval war three years before. The first of several revolutions had already occurred. The Czar was clinging to power with bloody crackdowns. And early on June 30, 1908, a few thousand scattered people in deepest Siberia witnessed a blinding, deafening blast that destroyed a vast region of taiga forest around the remote Tunguska River.
In Moscow, nobody cared. There was nothing out there, for one thing. There was no easy way to reach the Tunguska region, for another. And there was no money to send an expedition anyway to track down the outlandish stories from the deep woods.
At the end of that day, the first of several "white nights" affected northern Europe, so bright that you could read a newspaper all night. But those newspapers had nothing connecting this phenomenon to the event in Siberia. In fact, it took many years for Soviet scientists to reach the scene and reconstruct what had happened. It took many more years before all the facts reached the rest of the world, and longer still before we all realized just how important the Tunguska event is for the future of civilization. Today it is thought to have been a cosmic impact.
What's at Tunguska
The Tunguska event included seismic waves and infrasonic disturbances that were recorded around the world. Bright clouds and glowing skies were widely seen. But these also have terrestrial causes. The main evidence that points specifically to a cosmic impact is eyewitness accounts and the destruction at the scene.
Stories were collected from the inhabitants of the Tunguska region, and while not all of them agree, there was a general trend. A great searing light crossed the sky from south to north, followed by tremendous booms and shaking.
Around the central scene, more than 2000 square kilometers of forest was destroyed by fire and shock. Tree trunks were laid out flat, their tops pointing away from the central site. Detailed analyses suggest that a tight cluster of explosions occurred rather than a single one.
Unfortunately, no meteorites or large craters have yet been found to conclusively prove a cosmic impact, whether from an asteroid or a comet. (The differences are important: asteroids would be stone or iron bodies in relatively slow and predictable trajectories; comets would be icy bodies with high velocities coming from unexpected directions.)
In the century since 1908, new expeditions have returned to Tunguska again and again to collect evidence from ground zero. A recent Italian expedition, for instance, found what may be an impact crater that could hold a large fragment of an impactor. And more and more scientists are finding room for new ideas in the body of evidence, as this interesting Russian site demonstrates. And some scientists continue to hold that the cause of the Tunguska event was not from outer space but from a poorly known tectonic mechanism, perhaps similar to that of near-surface earthquakes and earthquake lights.
What Tunguska Means
Assuming that a cosmic impact is indeed what happened, the world was lucky in 1908. An impact of Tunguska's size could very easily have struck the ocean, where it would have raised a tsunami twice the size of those produced by the largest earthquakes. If it had struck a highly populated area, the deaths would have numbered in the millions, and the injuries even more.
Such events occur on Earth on the order of once a century, on the average. It may seem odd, therefore, that the Tunguska event is the only one that we have ever studied on the ground. Here's why it isn't so odd.
First of all, it's only in the latest two centuries that civilization has kept track of the whole world or anything near it. Before that, most parts of the world did not keep consistent, long-term records of natural events, and the oceans, two-thirds of the globe, were largely deserted.
Furthermore, the signs of an impact are not permanent. Craters form in some cases, such as Meteor Crater in Arizona, but in many cases, including Siberia, impactors explode far above the ground. At Tunguska, the airburst knocked down and burned trees over thousands of square kilometers, but fallen trees rot away and new ones grow within decades. The signs that remain would be quite subtle, and we haven't searched the whole planet yet that intensively. And of course an ocean impact would leave no craters either.
But there still are lots of human records to search. Records of sea level and tides have been kept for several centuries, and maybe there are impact-generated tsunamis to be found in them. And ancient writings have tantalizing hints of some very nasty events. Geologists have begun to argue, with archaeologists strongly dissenting, that geological events are just as good at destroying civilizations as famine, war and civil unrest. Bob Kobres's Comets, Culture, and Currency site is a first-rate exploration of the records of the oldest empires. These stories remind us how lucky we've been since those times.