"Graphic showing the Transition Zone inside the Earth Princeton seismologist Jessica Irving worked with then-graduate student Wenbo Wu and another collaborator to determine the roughness at the top and bottom of the transition zone, a layer within the mantle, using scattered earthquake waves. They found that the top of the transition zone, a layer located 410 kilometers down, is mostly smooth, but the base of the transition zone, 660 km down, in some places is much rougher than the global surface average. ""In other words, stronger topography than the Rocky Mountains or the Appalachians is present at the 660-km boundary,"" said Wu. NOTE: This graphic is not to scale. Credit: Kyle McKernan, Office of Communications"