What is the name of the deepest layer of Earth's mantle boundary with the core?

The D-double-prime layer, a couple of hundred kilometres thick, sitting on the liquid outer core about 2,900 kilometres down. The name is a leftover: an early twentieth-century seismologist labelled the Earth's shells with letters, and the D region was later split into D-prime and D-double-prime. Nobody has replaced it.

It is the strangest place in the planet. Seismic waves slow, speed up and split unpredictably passing through it, which indicates something structurally complicated - and the current picture includes a mineral phase change discovered in 2004, sitting alongside two continent-sized blobs of anomalous material under Africa and the Pacific whose composition nobody agrees on.

One proposal for those blobs is that they are the remains of Theia, the Mars-sized body that hit the early Earth and produced the Moon - denser material that sank and never mixed. It is also the boundary where plumes appear to originate, so the material that surfaces at Hawaii and Iceland may be arriving from a layer just above the core. It is 2,900 kilometres away, we can only see it with earthquakes, and it may contain another planet.