Earth's Inner Fort Knox

More than 99 percent of Earth's gold is missing—it all sank to the center of the planet billions of years ago. In fact, says geologist Bernard Wood of Macquarie University in Australia, there's enough gold in Earth's core to coat its surface in 1.5 feet of the stuff. How did it get there? Earth formed from a series of smaller planetesimals that crashed together over the course of 30 million to 40 million years. Wood deduced how much gold ought to be present in Earth's crust by comparing the crust's composition to that of meteorites similar to the planetesimals. He concluded that the crust was depleted of gold, platinum, and nickel and suggests that all these iron-loving elements were pulled into Earth's iron-rich core while its surface was still an ocean of molten magma. In fact, if meteorites hadn't later deposited gold on Earth's surface millions of years after its core had fully formed and its crust had cooled, gold would be even more rare and expensive than it is today. Wood has calculated that 1.6 quadrillion tons of gold must lie in Earth's core. This may sound like a lot, but it is really only a tiny percentage of the core's overall mass—about one part per million. The core holds six times as much platinum, Wood notes, "but people get less excited about that than gold."