A hundred years ago this week, a gigantic explosion ripped open the dawn sky above the swampy taiga forest of western Siberia, leaving a scientific riddle that endures to this day.

A dazzling light pierced the heavens, preceding a shock wave with the power of a thousand atomic bombs which flattened 80 million trees in a swathe of more than 2,000 square kilometres (800 square miles).

Evenki nomads recounted how the blast tossed homes and animals into the air. In Irkutsk, 1,500 kilometres (950 miles) away, seismic sensors registered what was initially deemed to be an earthquake. The fireball was so great that a day later, Londoners could read their newspapers under the night sky.

What caused the so-called Tunguska Event, named after the Podkamennaya Tunguska river near where it happened, has spawned at least a half a dozen theories.

The biggest finger of blame points at a rogue rock whose destiny, after travelling in space for millions of years, was to intersect with Earth at exactly 7:17 am on June 30, 1908.

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