Tunguska
1 A hundred years ago this week, a gigantic explosion ripped (撕裂) open the dawn sky above a forest in western Siberia, leaving a scientific riddle that endures to this day.
2 A dazzling light pierced the heavens, followed by a shock wave as strong as 1,000 atomic bombs. The explosion flattened 80 million trees across an area of 2,000 square kilometers. The fireball was so great that, a day later, Londoners could read their newspapers under the night sky. What caused the so-call Tunguska , named after the nearby Podkamennaya Tunguska river, still remains a mystery.
3 Experts suspect it was a rock that, after traveling in space for millions of years, was destined to crash to Earth at exactly 7:17 a. m on June 30, 1908. This possibility worries scientists. "Imagine an unspotted asteroid (小行星) hitting a significant chunk (块) of land...and imagine if that area, unlike Tunguska, were populated," the British science journal Nature commented recently.
4 But no fragments of the "rock" have ever been found. Finding such evidence would be important, for it would increase our knowledge about the risk posed by erous Near Earth Objects (NEOs), say Italian researchers Luca Gasperini, Enrico Bonatti and Giuseppe Longo. When the next Tunguska NEO approaches, scientists will have to decide whether to try to deflect (使偏转) it or blow it up in space.
5 However, several rival theories for the Tunguska exist. Wolfgang Kundt, a professor at Germany"s Bonn University, believes the Tunguska was caused by a massive escape of 10 million tons of methane (甲烷)-rich gas from deep within earth"s crust. Some people hold that the explosion was caused by an alien spaceship crash, or a black hole in the universe. Tunguska The hypothesis that the explosion was caused by a rock colliding with the Earth ______.