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Earths Inner Core Scientists have long struggled to understand what lies at the planets center, Direct observation of its center is impossible，so researchers must_____to other evidence. In 1889，a German scientist detected a severe earthquake in Japan.Geophysicists concluded that shock waves_____joltsfrom one side of Earth through the center to the other side.Then in 1936，Danish geophysicist Inge Lehmann studied the waves______ to determine that within Earths core of molteniron lies a solid inner core-but_____ that core was made of eludedher.Other geophysicists quickly determined that Lehmanns inner core was composed mostly_____iron. Since then， Lehmanns discovery has_____conventional Earth science. But now scientists are challenging traditional theory with new and radical _____ For example，Earths center could actually contain aninner core within the inner core，claim Ishii and colleague Adam Dziewonski. Analyzing hundreds of thousands of earthquake wave _____,they maintain that the inner core has at its heart a tiny, even more solid sphere . This spheremay be the oldest fossil____from the formation of Earth，says Dziewonski. Dziewonski and Ishii speculate that shortly_____ Earth formed around 4.8billion years ago，a giant asteroidsmashed into the young planet and nearly melted it. But Earths center didnt quite melt;it______mass as the planet cooled. The core within a core may be the kernelthat endured.Its presence could change our basic ideas about the_____ of the planet，Dziewonski says idea is tamecompared to the_____theories of independent geophysicist J.Marvin Herndon.Earths inner core is made not of iron，he claims but a_____of nickel and silicon Hemdon has a truly revolutionary notion：Within the nickel silicideinner core is also aninnerinner core-an 8 km-wide ball of the element uranium.Uranium is radioactive.Hemdon thinks the uranium releases heat energy as its atoms_____fission-split and crash into another in a chain reaction. In other words,we may live on top of a gigantic, naturalnuclear power plant. | 
