Scientists do not need to travel to Earth's centre to obtain evidence about its interior. Earthquakes send waves through the planet, and instruments record their arrival and motion. The distinction between kinds of waves is especially important: an absent signal is not automatically evidence of empty space, because different materials transmit different kinds of motion.
The US Geological Survey identifies four major layers: crust, mantle, outer core and inner core. Its explanation of earthquakes places the crust and the uppermost mantle in the moving surface plates. That is a layered model of a planet, not a thin inhabited shell surrounding a second open world.
USGS explains the critical difference between primary, or P, waves and secondary, or S, waves. P waves travel through both solids and liquids; S waves do not travel through liquids. Observations of those different paths revealed the liquid outer core. Consequently, the lack of a direct S-wave path through that region has a material explanation. It cannot be converted into evidence that no matter is present there.
The measurements are records of motion rather than photographs of the interior. USGS describes seismometers that compare ground movement with a mass that tends to remain still. Instruments recording separate horizontal directions and vertical motion provide more information than a single pendulum. Multiple stations are needed to determine an earthquake's position precisely.
Timing adds another constraint. P waves arrive before S waves, and the difference helps estimate the distance to the source. Waves can also change into other phases when they encounter boundaries inside Earth. The scientific account must explain these observations together: which signals arrive, when they arrive and how their paths change. A picture of an empty globe does not address those measurements.
The resulting conclusion is narrower and stronger than pretending every underground detail is known. Seismic observations support major internal layers and a liquid outer core. They do not support the proposed vast empty interior in hollow-Earth stories. Nor would evidence for an ordinary local cavity establish that planetary claim: the scale of the assertion is different from the scale of the feature. Claims about a hidden civilisation or polar entrance remain additional allegations, not deductions from seismic shadowing.
