The whole idea of the Tunguska fireball sounds like science fiction - but less believable than most. On 30 June 1908, a blindingly bright fireball flashed over Central Siberia, leaving an 800 km trail in the sky, then exploded with enormous force, creating a shock wave that levelled more than 2,000 km2 of forest. Locals witnessing the event believed the god of thunder had sent them a visitation.

The effects of the explosion were felt widely: 600 km to the southwest, the Trans-Siberian Express was shaken wildly on its tracks. Loud explosions were heard 1,200 km away. An observatory in Irkutsk recorded a magnetic storm, and microbarographs in England recorded clear peaks in atmospheric pressure about five hours after the explosion.

Across Europe, the night skies were strangely bright for several days.

Because of its remoteness, and political upheavals in Russia, the explosion site wasn't scientifically examined until 1927, when Leonid Kulik, an authority on meteorites, led an investigative team. He made a total of four expeditions to the area in search of evidence of a meteorite - all of them ultimately unsuccessful.

Ever since, the fireball has acted as a kind of Rorschach ink-blot test for the scientific community.

According to Surendra Verma, the author of The Tunguska Fireball, the most probable causes are either an asteroid or a comet. Other possibilities include a tiny black hole; a piece of mirror-, quark-, or antimatter; a volcanic blow-out; a lightning ball; and a plasmoid.

Less likely hypotheses include an exploding alien spaceship and a laser beam intended by aliens as a means of communication. Melbourne-based writer Verma works through each of the possible culprits, explaining their pros and cons, and giving unusually clear descriptions of all of the scientific phenomena mentioned.

Interesting side excursions into related issues include the history of scientific knowledge of asteroids and comets and the remaining questions about the extinction of the dinosaurs.