My guess is..
It's something to do with remnants of subducted oceanic crust. Once subduction begins, oceanic slabs become very dense, stripped of lighter material through partial melting. They also have a surprisingly large amount of momentum, and may sink all the way to the mantle/core boundary.
This site http://www.atlas-of-the-underworld.org/ has details of lots of named and identified slab remnants around the world, and the Hades Underworld Explorer even lets you produce a cross section of the mantle from tomography data, simply by drawing a line on the surface map. http://www.atlas-of-the-underworld.org/hades-underworld-explorer/.