Two hundred million years earlier, during the violent throes of planetary formation, Earth was a mass of molten minerals set afire by collisions with planet-sized heavenly bodies.
These Olympian crashes probably threw up the Moon and also caused billions of tonnes of liquefied gold and platinum -- enough to cover the planet with a crust four metres (13 feet) thick -- to sink to centre, creating its core.
And there the precious metals lie, forever beyond the reach of grasping human hands.
This much was known. The enduring mystery was why there remained anything more than trace amounts of gold in Earth's outermost skin, called the crust, and the layer underneath, the mantle.
Indeed, precious metals are tens, perhaps thousands of times more abundant in Earth's silicate mantle than they should be if none were added after the great meltdown during the planet's early phase.
Hence the theory that gold and other shiny rare metals were imported from outer space after our orbiting, rotating