On June 30, 1908 a colossal flash lit up the sky over Siberia, followed by an explosion with the power of a thousand atom bombs. It obliterated the taiga (forest) for hundreds of square kilometres (miles) in the basin of the river Podkamennaya Tunguska in the Krasnoyarsk region.

People living in the villages of Siberia thought there had been an earthquake. Humans and animals were thrown to the ground by the shockwave, windows were blown in.

No meteorite debris was found and scientists conclude that the core of a comet or an asteroid had exploded.

Researcher Yuri Lavbin has spent 12 years researching the mystery of the "Tunguska meteorite" and believes he has found the key to one of the great scientific enigmas of the last century, though many scientists remain sceptical.

He is president of the "Tunguska Spatial Phenomenon" Foundation in Krasnoyarsk, made up of some 15 enthusiasts, among them geologists, chemists, physicists and mineralogists, who have been organising regular expeditions to the area since 1994.

Lavbin's theory is that a comet and a mysterious flying machine collided 10 kilometres (six miles) above the earth's surface causing the explosion.

He and his team say that on an expedition to the Podkamannaya Tunguska river in July they found, between two villages, two strange black stones in the form of regular cubes with their sides measuring a metre and half (five feet).

These stones "are manifestly not of natural origin," Lavbin says. They appear to have been fired and "their material recalls an alloy used to make space rockets, while at the beginning of the 20th century only planes made of plywood existed."

He claims that the cubes are the remains of a flying machine, perhaps an extraterrestrial spaceship, while admitting that an analysis of the stones has yet to be undertaken.

He found something else: a huge white stone "the size of a peasant's hut" stuck in the top of a crag in the middle of the devastated forest.

"Local people call it the 'reindeer stone'. It is made of a crystalline matter which is not typical of this region," Lavbin said. He suggests it is part of the core of a comet.

The scientific establishment is not convinced.

"There are plenty of amateurs who organise trips to the site of the Tunguska cataclysm," said Anna Skripnik of the meteorites committee of the Academy of Sciences.

"In Siberia where oil geologists regularly work you can find a heap of fragments of various machines."

Lavbin is not deterred. He produces satellite photos of the region to back his theory which show the "footprints" of the spaceship (long marshes and lakes) and of the comet (devastated forests, charred trees and smashed rocks).

Not to mention a crater 500 metres (yards) across.

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