Submerged clue: Is Siberia's Lake Cheko the impact crater from a metre-sized fragment that survived the explosion?

Credit: University of Bologna

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Unique or regular?

"If the Tunguska event was in fact caused by a comet, it would be a unique occurrence rather than an important case study of a known class of phenomena," Gasperini's team wrote in last month's issue of Scientific American magazine. "On the other hand, if an asteroid did explode in the Siberian skies that June morning, why has no one yet found fragments?"

NEO experts are likewise unsure about the size of the object. Estimates, based on the scale of ground destruction, range from three to seventy metres across.

All agree that the object, heated by friction with atmospheric molecules, exploded far above ground – up to 10 km away. But there is fierce debate as to whether any debris hit the ground.

This information is also important. When the next Tunguska NEO looms, Earth's guardians will have to choose whether to try to deflect it or blow it up in space, with the risk that objects of a certain size may survive the fiery passage through the atmosphere and hit the planet.

The Italian trio believe the answers lie in a curiously shaped oval lake, called Lake Cheko, located about 10 km from ground zero. Computer models, they said, suggest it is the impact crater from a metre-sized fragment that survived the explosion.

They plan a return expedition to Lake Cheko in the hope of recovering a dense object of this size, buried 10 m below the lake's cone-shaped floor. Something fitting that description has already been detected with sonar waves.

Rival theory

But what if neither comet nor asteroid were to blame? A rival theory is given an airing in New Scientist magazine.

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 km², Kundt said.