Was it a gigantic meteorite? A tremendous bolt of lightning? Perhaps the crash of a UFO the size of Tokyo?

No one is certain of the answer to one of the 20th century's greatest scientific mysteries -- the "Tunguska Event" 100 years ago this week.

But a group of maverick Russian scientists gathered in Moscow this week left no doubt that they share a singular passion to find out what caused the huge explosion in a Siberian forest that lit up the night sky as far away as London.

"The facts collected over 100 years disprove the hypothesis of a meteorite or comet. The sooner we understand that the better," said physicist Boris Rodionov to applause from the around 30 scientists at the conference.

"If it was just a meteorite, we wouldn't be sitting here 100 years later," Rodionov told the conference, held exactly a century after the June 30, 1908 explosion, which destroyed a vast swathe of Siberian forest.

The most commonly held theory is that the blast -- hundreds of times more powerful than the atomic bomb dropped on Hiroshima -- was caused either by the impact of a meteorite or by its explosion above the Earth's surface.

But some Russian scientists, many of whom have travelled to the site 4,000 kilometres (2,485 miles) east of Moscow, argue that does not make sense since there are no fragments of the meteorite and no crater from the impact.

A conference devoted to the "Tunguska Event" last year nearly came to blows between the "meteoreticians" and the "alternativists," said Andrei Olkhovatov, an amateur scientist with a doctorate in physics and an expert on Tunguska.

"The meteorite theory is the main one. We're like the poor relatives."

This year, the "alternativists" organised a separate conference held in a museum on Moscow's picturesque Old Arbat street at which they sketched out outlandish theories for an event they say ordinary physics cannot explain.

Rodionov said the explosion was most likely caused by US physicist Nikola Tesla (1856-1943) detonating an underground volcano in Siberia by harnessing electric charges in the air from his laboratory tower outside New York.

Other theories outlined at the conference ranged from the thesis that it was a particularly powerful bolt of lightning to the proposition that it was the result of interaction between yin and yang energy fields in the universe.

As he pointed to multi-coloured drawings of lightning coming from the Earth's core, Vladimir Mikhailov, a clairvoyant with an intense stare, said: "My theory explains everything. I just needed a place to express myself."

This week marks the culmination of a series of events devoted to the phenomenon, including a conference in the village of Vanavara, the settlement nearest to the epicentre of the blast near the Tunguska River in Siberia.

Photographs published in the Komsomolskaya Pravda daily on Monday showed that some of the 80 million trees felled by the explosion over an area of some 2,000 square kilometres (770 square miles) are still visible today.

Researchers at the conference on Monday said the enduring popularity of the meteorite theory was only due to the fact that there is more money for scientists in stressing the danger of meteorites for the Earth.

"It's all linked to financing. They are just attracting attention to the danger of meteorites, using the example of Tunguska. It worries people," said Sergei Sukhonos, author of several books on physics.

"There are no answers to our questions. It can't be explained by traditional physics.... There are always new theories coming up, there are about 100 theories. No one knows the truth. We have to be patient."