The Huge "Blobs" in the Mantle 

A correspondent has sent me THIS LINK which is a very approachable introduction to a topic which has appeared in this blog several times over the years.

The existence of the blobs was found by seismologists and a lot of what we know of them comes from their work. Experimental petrologists have tried to replicate the physical conditions in their laboratories.

But we still know very little about them. The article goes through the various theories. The most intriguing is that the blobs are the remnant of Theia, the planet which crashed into the Earth, producing the moon. An interesting theory, rather difficult to prove.