text: According to the experts, it's a solid iron ball.  That's where we get the Earth's magnetic poles from.  (BTW, the magnetic poles have been known to flip--usually about every 130,000 years, but they haven't for about 700,000 years [we're overdue!].)  If we didn't have the magnetic poles caused by the Earth's core, the atmosphere would be stripped away, like Mars'.  And, without the magnetic field around the Earth, deadly levels of radiation from the sun would fry everything on the planet.  Not good.