Deep within the Earth, far below the layers of rock that form the crust, and even further below the molten rock of the mantle, lies a hot core of Iron and Nickel.  The swirling of the liquid metal creates a flow of charge and produces the magnetic field of the Earth, without which we humans could not survive. But there is still more.  At the centre of the Earth, a part of the liquid metal core, the size of Pluto, cooled into a solid ball of Iron and Nickel.  When in the Earth’s history did it form? This question has...