Tunguska Revisited

Hoping to solve a mystery that has persisted for more than 90 years, an Italian-Russian team last month scoured the bottom of a Siberian lake for clues to the cause of a titanic explosion that flattened and charred trees over hundreds of square kilometers.

On 30 June 1908, witnesses saw a fireball hurtle through the dawn sky over Siberia before exploding with a force of 1000 Hiroshima bombs in a remote region, called Tunguska, in the middle of eastern Siberia. Various theories on the cause of the event have flourished as Russian expeditions to the site over the years have failed to turn up credible fragments of a meteorite or other object.

Eight years ago, a group led by physicists Giuseppe Longo and Menotti Galli of the University of Bologna in Italy culled microscopic spherical particles from 90-year-old tree resin at the site. The particles, containing elements like copper, gold, and nickel, appeared to have been forged at high temperatures. But scientists don't agree on what they could mean. Western researchers tend to think the culprit was a meteorite, while many Russians opt for a chunk of cometary debris. Comets are mostly dirty snowballs, but some are thought to have heavy metals in their cores.

Now Longo and Nikolai Vasiliev, deputy director of the Tunguska Nature Reserve, have returned to Siberia to look for more definitive evidence. Braving mosquitoes, horseflies, and sweltering heat, the scientists, working from a makeshift catamaran, took two dozen core samples from sediments at the bottom of a lake 8 km from the epicenter, where, they figure, any fragments should be preserved in the sediment. Longo's group will analyze the cores in Italy, hoping to find more particles--or perhaps larger fragments of a meteorite--that might solve the puzzle (www-th.bo.infn.it/tunguska).