Searching for a pot of gold? Try the center of the Earth. 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...