BBC地平线系列：地心

For centuries we have dreamt of reaching the centre of the Earth. Now scientists are uncovering a bizarre and alien world that lies 4,000 miles beneath our feet, unlike anything we know on the surface. It is a planet buried within the planet we know, where storms rage within a sea of white-hot metal and a giant forest of crystals make up a metal core the size of the Moon. 　　Horiz...