What’s going on? One idea is that two titanic forces are battling for control over the world’s heart. Earth’s magnetic field, generated by swirling iron currents in the liquid outer core, is pulling at the inner core, causing it to spin. That impulse is countered by the mantle, the mucilaginous layer above the outer core and below Earth’s crust, the immense gravitational field of which grasps the inner core and slows its spin.
By studying core-diving seismic waves recorded from the 1960s to the present day, Dr. Song and Yi Yang, another Peking University seismologist and a co-author of the study, posit that this tremendous tug of war causes the inner core to spin back and forth on a roughly 70-year cycle.
In the early 1970s, relative to someone standing on Earth’s surface, the inner core was not spinning. From then, the inner core has gradually spun faster eastward, eventually overtaking the speed of rotation of Earth’s surface. Afterward, the inner core’s spin decelerated until its rotation appeared to have stopped at some point between 2009 and 2011.
The inner core is now starting to gradually spin westward relative to Earth’s surface. It will likely accelerate then decelerate once again, reaching another apparent standstill in the 2040s and completing its latest eastward-westward spin cycle.
This 70-year rhythm, if it exists, could have a tangible effect on parts of Earth’s deeper viscera. But it may only be capable of stirring up comparatively minor turbulence closer to the surface — perhaps by causing subtle shifts in the planet’s magnetic field, or even by very slightly tweaking the length of a day, which is known to go increase and decrease by a fraction of a millisecond every six years.
This is just one of several competing models explaining the erratic voyages of waves that reach the core. It is also possible that Earth’s innermost layer is wobbling about. Conversely, Earth’s ferrous nucleus may have a metamorphosing surface, twisting any seismic waves that pierce it. “No matter which model you like, there’s some data that disagrees with it,” Dr. Vidale said.
Because of its inaccessibility, this abyssal realm may forever elude explanation. “It’s certainly possible we’ll never figure it out,” Dr. Vidale said. But, he added, “I’m an optimist. The pieces are going to fall into place someday.”