When Russian mineralogist Leonid Kulik made his way to the remote Tunguska River basin, his group found thousands of pine trees lying burned in a radial pattern.

JUNE 30, 1908.

Reindeer graze beside the Podkamennaya or Lower Stony Tunguska River that winds through the Siberian steppes.

The tents of a few herdsmen stand nearby, but hardly anyone else lives in this land of swamps and forests.

7.14 am. Pine trees glow in the summer light. The morning is blue and cloudless.

Then a blinding ball of light rips across the sky, trailing a column of fire.

Some eye-witnesses say the light was red. Others claim it was blue, and cylindrical in shape.

It races down towards the Tunguska River, and explodes.

A spear of fire splits the sky. Explosions boom across the land.

A dark mushroom cloud begins rolling upwards. It will reach a height of 80km: ten times higher than Mt Everest.

Over 70km away, a farmer is in his yard when the sky to the north bursts into fire.

A wall of heat strikes him; his shirt is almost burned off. A blast of air hurls him across the yard, and noise bellows all around.

House windows shatter; the building rocks.

Nearly 200km from the explosion, another farmer in a field hears several deafening bangs.

The pine forest thrashes. He and his horse are almost knocked off their feet. Soil flies into the air.

A wall of water is blown up the nearby river.

Elsewhere, herdsmen's tents are whipped from the ground by shock waves.

Only two people die, but near the explosion site, thousands of reindeer are burned to death.

A fireball brighter than the Sun can be seen from 1000km distant.

In France and Belgium, sunsets and sunrises are brilliant red for a week.

At night, clouds glow greeny-white, so bright that people can read a newspaper at 3am. So what caused what's now cautiously labelled The Tunguska Event?

A century later, the arguments continue.

In the years following 1908, Russia was torn by World War 1 and revolutions.

It wasn't till 1921 that Russian mineralogist Leonid Kulik made his way to the remote Tunguska River basin on a survey for the Soviet Academy of Sciences.

For two months, his group struggled through forests and across swamps, tormented by mosquitoes.

They reached a high plain, and gaped. Thousands of pine trees lay burned in a radial pattern.

At the centre, where the explosion seemed to have occurred, the ground was covered with swamps and scummy ponds.

Kulik urged the Government to send a bigger expedition to the area, to search for iron from what he felt sure was a meteorite impact.

In 1927, the requested party arrived at what seemed to be Ground Zero, puzzled by the absence of a crater.

They drilled into the swampy ground and found nothing. Nothing except mud, dead wood, and tiny blobs of melted rock.

In the decades since, scores of scientific and less-scientific missions have headed to Tunguska.

According to Surendra Verma in The Tunguska Fireball, the consensus of opinion now holds that the explosion of June 30, 1908 was caused by an asteroid or comet (a fragment from short-term Comet Encke, say some).

The object exploded 5km-10km above the ground with a force estimated at 10-15 megatonnes, a thousand times the force of the bomb dropped on Hiroshima. A 1938 aerial photographic survey estimated that some 80 million trees had been felled over a 2000sqkm area.

In the 1960s, Soviet nuclear airburst tests produced the same pattern of pines directly below the explosion still standing but with branches stripped off, while others more distant lay pointing away from the blast.

So, an aerial blast.

Comet or asteroid? Eye-witnesses described the fireball entering Earth's atmosphere on a shallow trajectory.

An ice-and-rubble comet should have disintegrated almost immediately, whereas the Tunguska object apparently remained intact till just above the surface.

On the other hand, those bright nights following the explosion fit the scenario of a trail of comet dust dispersed through the upper atmosphere.

If the object exploded just a few kilometres above the surface, the downward blast should have formed a crater.

Nobody has ever found a crater at Tunguska.

Then in 1999, an Italian expedition to Lake Cheko, 8km northwest of the explosion site, suggested that the lake's bowl-shaped crater was just the right shape for such an explosion.

Moreover, their instruments seemed to show a massive object buried beneath the lakebed.

And Lake Cheko doesn't appear on any maps before 1929. Promising but inconclusive. No underground object has been found.

Dissenting voices argue that the lake doesn't fit the classic crater shape at all.

Plus nearby, century-old trees don't stand the way they should beneath a blast site.

But should there actually be a crater? Computer simulations suggest that a stony body 60m across - the right size for such an explosion - would build up such pressure and temperature as it bored through the atmosphere that it might blow itself apart, leaving no crater but causing blast and thermal damage over a wide area.

Others have suggested more lurid causes for the fireball, flash and explosion.

Did a microscopic black hole, the sort formed in the first seconds of the universe, speed through our planet, entering at Tunguska?

Problem: where's the exit hole? Did a speck of anti-matter hit Earth? Problem: no sign of nearby gamma-ray bursts, which would be the signature of such an event.

Was it a ball of lightning? A blob of white-hot gas hurled from the Sun? A doomed alien spaceship? A laser ray from a distant planet?

Enter Tunguska Conspiracy on the web, and you'll see these are among the more restrained suggestions.

Another theory suggests the explosion wasn't caused by something from above, but by something from below.

Underneath the Tunguska area lie big deposits of oil and natural gas.

Did gas seep upwards through a crack in the ground and then explode in the great fireball? Such gas would probably include methane, which burns very easily.

The gas might keep burning for several days, which would explain the bright clouds that people kept seeing at night after the explosion.

But this theory doesn't explain the blazing object that so many people saw hurtling down the sky.

The Tunguska Event isn't the only one of its kind.

In the past 100 years, nearly 20 aerial explosions of apparently extraterrestrial origin have been reported, the most recent occurring high above the Mediterranean in June 2002.

But the largest of these others, over the Amazonas region of Brazil during August 1930, was estimated at about 500,000 kilotonnes.

The Tunguska explosion was 25 times more powerful.

Not suprisingly, it has given further weight to demands for the development of an asteroid deflection system.

Picture the Tunguska Event happening now over New York or London.

Actually, don't.