Recent studies of the D'' layer have revealed parts of it to be a graveyard of sunken slabs of oceanic lithosphere that have slowly drifted down through the mantle to come to rest on the liquid core. These slabs have piled up to form anti-continents that are the opposites of the ones we dwell upon, having sunk to the bottom of the mantle instead of floating on top. Here then is the physical manifestation of Tartarus, the mythic realm below Hades, where the first gods were banished by the gods of Olympus. This is a land where anti-continents float on liquid metal underneath a silicate sky, bordered by seas of magma from which rise 2,000 mile long columns of molten rock connecting that shadow realm with ours. The outer core is known to be molten, because S waves do not pass through it, and is believed to flow in huge convection cells that are an important part of the geodynamo that generates Earth's magnetic field.
P waves move measurably faster through the inner core in a north-south direction than across it.
In fact, many geophysicists believe the inner core is a single gigantic crystal of iron. Talk about your crystal magic, Mama's got a big one. Either way, one crystal or many, the inner core has crystallized with its principal crystallographic axis oriented nearly north south. Consequently, the seismic waves that follow this axis arrive sooner than would otherwise be expected if the inner core were made up of randomly oriented crystals. Furthermore, the maximum seismic-velocity axis is tilted about 10 degrees relative to Earth's rotation axis, allowing seismologists to identify a slow eastward motion of this feature, such that it rotates once every 400 years relative to Earth's surface.