London, June 28 : The Tunguska event, which was a massive explosion that flattened millions of trees in the Siberian wilderness in 1908, has completed its 100-year anniversary, with the mystery as to what caused it, still unsolved.
According to a report in The Times, a century later, the Tunguska event still provokes intense debate over a blast 1,000 times more powerful than the Hiroshima bomb.
Most argue that the explosion was a comet or asteroid smashing into Earth.
A lack of physical evidence - not least a credible impact crater - has led others to contend that the cause was a sudden eruption of gas from deep within the surface of Earth.
Some even argue that an alien spacecraft blew up, or that a black hole made a freak appearance.
The fireball levelled an estimated 80 million trees in a radial pattern over 2,000sq km. People 40 miles away were bowled over by a shock wave and the sky across Europe was lit that night by an ethereal white-and-yellow light.
The full immensity of what had occurred became apparent only when Leonid Kulik, a Soviet meteorite expert, led an expedition to the site in 1927.
Witnesses among the local Evenki tribe recalled a searing light and deafening thunder as the ground shook.
It was generally assumed that the event was caused by an asteroid.
Later comparisons with tree patterns after Hiroshima led some to speculate that there had been a nuclear blast from a UFO.
On June 26, three Italian experts told the Russian Academy of Scientists that Lake Cheko, five miles from the epicentre of the blast, was the missing impact crater.
In a joint study with Russian scientists, they argued that the flattened trees showed that "two bodies entered the atmosphere. One exploded about five miles above ground, while the other hit the Earth where Lake Cheko is now."
Others argue that there is evidence that the lake existed long before 1908.
According to Jason Phipps Morgan, a geophysicist at Cornell University in New York, a giant "Earth burp" may have caused the devastation.
With the mystery still unsolved, the local inhabitants are marking the anniversary of the explosion by unveiling a statue to the Evenki god of thunder.