Scientists now think they know why seismic waves traveling between the Earth's poles move faster than those traveling east-west. It has to do with the way that the iron alloys at the planet's core have crystallized. While in in the past we thought of the earth's core as fairly homogeneous, it's now clear that the way the core is set up is a little more complicated. “The center of the earth is literally a crystal,” said David Stephenson, a geologist at CalTech. Over time, the crystal grew, and now it is an aggregate of them.