The Italian trio believe the answers lie in a curiously-shaped oval lake, called Lake Cheko, located about 10 kilometres (six miles) from ground zero.

Computer models, they say, suggest it is the impact crater from a metre- (three-feet) -sized fragment that survived the explosion.

They plan a return expedition to Lake Cheko in the hope of reaching a dense object of this size, buried 10 metres (32.5 feet) in the lake's cone-shaped floor, that reflected sonar waves.

But what if neither comet nor asteroid were to blame?

A rival theory is given an airing in this week's New Scientist.

Lake Cheko does not have the typical round shape of an impact crater, and no extraterrestrial material has been found, which means "there's got to be a terrestrial explanation," Wolfgang Kundt, a physicist at Germany's Bonn University told the British weekly.

He believes the Tunguska Event was caused by a massive escape of 10 million tonnes of methane-rich gas deep within Earth's crust. Evidence of a similar apocalyptic release can be found on the Blake Ridge on the seabed off Norway, a "pockmark" of 700 sq. kms (280 sq. miles), Kundt said.