Today in 1908, an enormous explosion occurred near the Tunguska River in Siberia. The blast had the equivalent energy of between 10 and 15 million tons of TNT, but while the available evidence points to a likely cause of the event, no conclusive proof has ever been made public. More than a century after it took place, the Tunguska event remains one of the greatest mysteries of the 20th century.
Central Siberia was, and remains today, a remote wilderness. Even so, there were witnesses to the event. Near Lake Baikal, villagers saw a bluish light move across the sky at 7:15AM; they described it as being brighter than the sun. 10 minutes later, there was a bright flash and a sharp noise that sounded like artillery fire. When the shock wave arrived, it knocked people off their feet and broke windows. It was so strong that people felt its force hundreds of miles away. In England, barographs, used to measure atmospheric pressure, showed fluctuations from the explosion. That night, and for weeks thereafter, night skies in the northern hemisphere contained a strange glow so bright it allowed people to read outside. In the United States, the Smithsonian Astrophysical Observatory and the Mount Wilson Observatory observed a decrease in atmospheric transparency that lasted for several months, believed to have been caused by dust suspended high in the atmosphere.
The remoteness of the Tunguska area coupled with the chaos present in Europe during World War One, the Russian revolution of 1917 and the subsequent civil war meant that a scientific expedition did not study the explosion site until 1921. What the researchers found was astonishing. At what should have been the impact point for whatever fell out of the sky, a stand of trees stood with their branches and bark stripped. Outward from there, for a distance of nearly 40 miles, every tree was laid flat. Aerial photos made of the area in 1938 showed that the trees were knocked down in a sort of butterfly-shaped pattern. Still, there was no crater to be seen. Whatever came to earth that day seemed to have exploded in mid-air.
Further expeditions to the site found tiny glass spheres embedded in the soil. The spheres were found to contain iridium and nickel in close proportion to the concentrations found in some meteorites. These findings led scientists to conclude that the most likely cause of the explosion was a meteoroid which exploded several miles above the Earth’s surface. This would explain the lack of a crater and the upright trees found at ground zero. Model testing concluded that the butterfly-shaped damage pattern could have been caused by an object approaching the Earth at a 30 degree angle.
Of course, not all researchers agree on what happened at Tunguska. One competing theory states that the object was a comet. Comets are composed mainly of dust and ice, so one exploding high in the atmosphere would leave no trace and would help explain the bright night sky observed after the explosion. Geologists have discovered that the region in question was once very volcanic. Therefore, a theory has been put forward stating that nothing fell from the sky, but rather a large pocket of methane from the Earth exploded. This type of explosion has happened on a smaller scale at other locations around the globe, so it is not implausible.
From there, the hypotheses grow more and more incredible. Two researchers put forth a theory in 1973 that a small black hole passed through the Earth and caused the damage. If true, then there would have to be an “exit wound” on the other side of the Earth; none has been found. Three scientists hypothesized in 1965 that the explosion was caused by a chunk of anti-matter. What the scientists failed to explain was how the anti-matter got so close to the Earth without being annihilated by regular matter higher in the atmosphere. Ufologists have long theorized that an alien spacecraft could have caused the incident.
The fact that Tunguska is located in an area that was once part of the Soviet Union made study of the region even more difficult after the Second World War as mistrust of Western researchers grew. Some of the photographic evidence collected by Soviet scientists in the late 1930's was destroyed 40 years later, possibly because of professional rivalries inside the Communist state. By the time the USSR collapsed at the end of 1991, the Tunguska event had become a dim memory among scientists and even those interested in the mystery found the prospect of traveling to central Siberia daunting.
But research has continued. In 2007, scientists from the University of Bologna identified Lake Checko, a small body of water in the Tunguska region about five miles from the center of the event, as a possible impact crater from the event. They do not dispute that the Tunguska object exploded in midair but believe that a one-meter fragment survived the explosion and impacted with the ground. This hypothesis has been disputed by other impact crater specialists for 50 years. A 1961 investigation dismissed the theory, saying that the presence of deep silt deposits on the lake bed suggest an age of at least 5,000 years, but recent research concluded that only a few feet of the sediment layer on the lake bed is "normal”, a depth indicating a lake of possibly only 100 years in age. Soundings revealed a conical shape for the lake bed, which is consistent with an impact crater. Magnetic readings show a possible meter-sized chunk of rock below the lake's deepest point that may be a fragment of the object. Finally, the lake's long axis points to the center of the Tunguska explosion. After two years of research, the University published the scientists' findings, concluding that the lake was created by a meter-long chunk of rock that had to have come from whatever exploded over the Tunguska region in 1908. As of this writing, no sample of that rock has been retrieved.
Eugene Shoemaker of Shoemaker-Levy fame estimated that explosions like that which took place over Tunguska occur every 300 years. If this is accurate, then it is important to understand exactly what happened that morning over a century ago because it will happen again, be it tomorrow or centuries from now.