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Scientists have long struggled to understand what lies at the planets center. Direct observation of its center is impossible, so researchers must （1） to other evidence. In 1889, a German scientist detected a severe earthquake in Japan. Geophysicists concluded that shock waves （2） jolts （晃动） from one side of Earth through the center to the other side. Then in 1936, Danish geophysicist Inge Lehmann studied the waves （3） to determine that within Earths core of molten （熔化了的） iron lies a solid inner core － but （4） that core was made of eluded （难倒） her. Other geophysicists quickly determined that Lehmanns inner core was composed mostly （5） iron. Since then, Lehmanns discovery has （6） conventional Earth science. But now scientists are challenging traditional theory with new and radical （7）. For example, Earths center could actually contain an inner core within the inner core, claim Ishii and colleague Adam Dziewonski. Analyzing hundreds of thousands of earthquake wave （8）, they maintain that the inner core has at its heart a tiny, even more solid sphere （球体）. This sphere may be the oldest fossil （9） from the formation of Earth, says Dziewonski. Dziewonski and Ishii speculate that shortly （10） Earth formed around 4.8 billion years ago, a giant asteroid （小行星） smashed into the young planet and nearly melted it. But Earths center didnt quite melt; it （11） mass as the planet cooled. The core within a core may be the kernel （核心） that endured. Its presence could change our basic ideas about the （12） of the planet, Dziewonski says. | 
