means the layered structure beneath earth's surface, from crust to mantle to liquid and solid core, that drives everything from earthquakes to the magnetic field.

from inferred not from digging (deepest hole ever is 12 km, a pinprick) but from seismic waves — earthquakes send vibrations through the planet, and how those waves bend and vanish revealed the hidden layers, mostly worked out in the early 20th century by seismologists reading the planet's own tremors like an ultrasound.