An Italian expedition has begun the most detailed on-site survey yet of the aftermath of the most powerful and devastating explosion on Earth this century.

The scientist are braving heat, humidity and swarms of biting insects in Tunguska, a remote part of central Siberia. Here, in 1908, a huge explosion caused widespread destruction but no-one yet knows whether the culprit was a comet or a meteorite.

The force of the explosion was equivalent to hundreds of Hiroshima-scale bombs and felled perhaps a hundred million trees . Strangely no impact crater was formed, possibly because the object exploded in mid-air.

Middle of nowhere

Since then there have been only a few expeditions to Tunguska of which the current one is the most sophisticated.

Exploring Lake Ceko

The team, from the University of Bologna in Italy, reached Tunguska about ten days ago after days of aircraft and helicopter flights from Moscow.

Progress reports and photographs are sent to the outside world via the Internet which they access via satellite.

The scientists established a base camp on a tongue of land protruding into a lake, as this is easier to defend from wild bears. They next began exploring Lake Ceko, the largest body of water near to the explosion site.

Trek to the epicentre

They intend to drill into the sediments on the bed of the lake and hopefully collect samples of the object that exploded in 1908.

Trailing sonar sounders and other sensors behind a catamaran they mapped the lake bed showing that Lake Ceko is a funnel-shaped lake with a depth between 54 and 56 metres.

Penetrating radar measurements mapped some of the sediments in the top 100 metres of the lake bed, much of which is covered with felled trees. Fortunately they have identified several regions of clear bottom in which to drill.

An automated submarine TV camera has shown vegetation strewn on the lake bed to a great depth.

Siberian summer

Conditions in central Siberia are difficult. Mid-summer temperatures can reach 36 degrees Celsius. The researchers say there are fewer mosquitoes than during the previous expedition in 1991, but many more horse-flies.

One of the main goals of the expedition is to make measurements at the point on the ground that was directly under the explosion. Reaching that spot involved a 12-kilometre trek through marshes and dense forests.

Samples were taken from the ground and from the bark of living trees, as well as trees killed during the explosion. It is hoped that material from the exploded comet may be recovered.