30 June ,1908.A mysterious explosion 3 miles above the Podkamennaya Tungusta River basin ,flattening 3900 sq m of trees.

The object, whatever it was, appeared above Western China, heading due north, just after seven in the morning. It plunged through the atmosphere glowing with the heat of 5,000 degrees. In central Russia it moved overhead with a deafening supersonic roar that terrified the inhabitants. Before the object raced a ballistic wave that leveled trees and knocked over houses.

Then at 7:17 AM, near the Stony Tunguska River, a cataclysmic explosion occurred. It was so powerful that the seismograph at Irkutsk, some 550 miles away, registered what looked like an earthquake. Even in Washington D.C., on the other side of the world, the shock was recorded by sensitive seismic devices.

Forty miles from the blast center at a town called Vanavara, people were thrown into the air by a shock wave that shattered windows and collapsed ceilings. Herdsmen working closer to the site were deafened by a series of thunderclaps that could be heard for 500 miles. Near the town of Kansk, a stop on the Trans-Siberian Railway, a train screamed to a halt when the engineer feared it would be thrown from its tracks by the violent shaking. Passengers were jolted from their seats by the movement. Kansk was 375 miles from the blast center.

As the shocks settled down the whole region around the Tunguska was showered with “black rain”: condensation mixed with dirt and debris sucked into the swirling vortex of the explosion and thrown out again. Amazingly the blast point was so isolated that there was no record of any human being dying at Tunguska despite an explosion that would have dwarfed the bombs dropped at Hiroshima (above-left) and Nagasaki during WWII. So isolated that no scientist bothered to investigate the rumors of the event for thirteen years.

Then in 1921 a Russian scientist named Leonid Kulik was charged with the task of locating and examining meteorites that had fallen within the Soviet Union (Meteorites are rocks in space, sometimes refered to as asteroids, that fall to Earth. If they burn up as they hit the atmosphere they are called meteors). While preparing for the expedition he came across an account of the Tunguska explosion reprinted from an old newspaper. It took him six years to finally find the site of the blast.

On April 13, 1927, Kulik stood on the edge of the Makirta River and looked out across the land at the immense devastation. “The results of even a cursory examination exceeded all the tales of the eyewitnesses and my wildest expectations,” he wrote.

Kulik discovered an oval more than 40 miles wide where the forest had been flattened (above-right). Trees were uprooted, burned and laid with their tops pointing out from the heart of the affected region. At the center Kulik expected to find a large crater where the meteorite had hit along with fragments of the meteorite itself. He didn’t.

Instead of finding a crater in the center, Kulik found a very strange forest. Trees here were not pushed over and uprooted. Instead they stood straight up like telephone poles and were stripped of their branches. A careful search of the area also yielded no remnant of any meteorite.

Kulik continued to look for the meteorite, unsuccessfully, for the rest of his life, but other scientists began to think that perhaps the object was something else. In the early 1930s two astronomers, F.J.W. Whipple and I.S. Astapovich independently came to the conclusion that the object had been a gaseous comet ( below-left) that had left no trace of itself after impact. Still, if a comet hit the ground, where was the crater?

In the 1940’s a Russian scientist named E.L. Krinov, who had traveled to the site on one of Kulik’s trips, suggested there was no crater because the object must have exploded before hitting the ground. Unfortunately further work on Krinov’s theory had to be put off. The Soviet Union became engaged in defending itself from Germany during WWII and there was little time or money available for scientific research not connected with the war effort.

The war brought with it a new, terrible weapon. The weapon’s use, though, brought new insight on the Tunguska explosion. Aleksander Kazansev was one of the first Russian scientists to evaluate the atomic bomb explosion at Hiroshima, Japan. He was also intrigued by the mystery of the Tunguska blast and quickly found connections between the two. The strange forest of trees, stripped of branches, but still standing, was found at Hiroshima too. The American atomic bomb had exploded at high altitude and the downward rushing shock wave had left the trees directly beneath standing while flattening trees, and houses, further out in a radiating pattern. The mushroom shaped cloud and the black rain that followed the Hiroshima blast also conformed to reports from Tunguska.

Kazantsev was the first to suggest that the event was the caused by the explosion of an atomic powered spaceship. While most scientists laughed at this explaination, some took seriously his suggestion the blast was atomic in nature, and they began to notice other similarities. This included apparent effects from radiation. Both the reindeer population at Tunguska and the human population at Hiroshima developed similar skin diseases. There was also evidence of accelerated plant growth at both locations.

Some scientists suggested that if the blast was atomic in nature it might have been explained by some natural phenomenon, rather than a spaceship. Two possible candidates are an anti-matter meteorite or a mini-black hole.

Anti-matter is material with a reversed charge at the sub-atomic level. As far as we know it is extremely rare in the universe, but it has been produced on Earth in laboratory experiments. When anti-matter meets up with normal matter they annihilate each other in a burst of energy. A small chunk of anti-matter could make an enormous explosion. If the Tunguska meteor was made of anti-matter it would have exploded violently when it came into contact with the thick, lower atmosphere. The effects of the explosion would have looked very much like that of an atomic bomb.

One objection to the “anti-rock” theory is that an anti-matter explosion should have set off a chain of events leading to a significant rise in the amount of radioactive carbon-14 in the air. Scientists examined tree rings laid down in 1908 and found a rise in carbon-14, but not enough to support the idea of an annihilation large enough to explain the explosion.

“Black holes” are usually the final result of the collapse of a large star at the end of its life. Some cosmic theories suggest, though, that “mini- black holes” might have been created at the beginning of the universe and drift aimlessly through galactic void. The density and resulting gravitational pull of such an object is so high that not even light can escape from it. A mini-black hole passing through the Earth certainly would have produced many of the effects seen by witnesses of the Tunguska event. What it would probably not have produced is the visible sighting as seen as the object entered the atmosphere. A black hole should have also produced a crater. For these reasons, argue some scientists, a mini-black hole is not a serious contender to explain what happened.

In the 1960’s several Soviet scientists tried to show that the object had changed course during its descent. They based their theory on eye-witness accounts and ground damage from the ballistic shock wave that proceeded the object as it traveled at supersonic speeds. This idea, though, has not been completely accepted in scientific circles, but if the object did make a mid-flight maneuver it would certainly bolster the spaceship theory.

Research on Tunguska continues today. Christopher Chyba, of NASA, Paul Thomas, of the University of Wisconsin, and Kevin Zahnle, of NASA, have used computer simulations to calculate that a stony asteroid, about 100 feet across, could produce the right sized blast when it fragmented and vaporized before hitting the ground. They point out that iron asteroids are much more dense and would have probably survived the flight to hit the ground and make a crater. Recent analysis of resin from trees surviving the blast seem to confirm the Chyba-Thomas-Zahnle theory.

So what was the Tunguska explosion? Meteorite, comet, anti-rock, mini-black hole or alien spaceship? It remains a mystery. One thing is for sure, if the object had hit the Earth a few hours later it would have come down over Europe and the death toll would have been a half-million people.