Far below your feet—about 3,219 miles down—is Earth's other moon. Rotating independently of the planet, turning at a different speed within a fluid outer core, this solid, satellite-size sphere holds clues to understanding Earth's earliest history and perhaps even life on the planet. For 300 years, little was added to the pos-tulation that Earth had at its center a homogeneous ball of pure iron. But in the past decade or so, geophysicists, using new data and laboratory simulations, have started to map and analyze it. Every year, more features are discerned from seismic evidence. Scientists don't yet agree on what all the information means, but one thing is becoming dear: Earth's center is far more varied and unusual than anyone had previously thought.
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