The ground beneath your feet may feel solid, but Earth is constantly shifting. While we have mapped the surface in detail, its deep interior remains a mystery. Even with modern technology, no probe has ever reached beyond the thin crust, which is only about 35 kilometers deep. To study the Earth's core and mantle, scientists must rely on indirect methods.
From space, Earth appears as a smooth blue sphere, but its shape is far from perfect. Beneath the surface, unevenly distributed mass creates gravitational variations, distorting its form. The movement of tectonic plates further reshapes the planet, building mountains, carving valleys, and adding to its irregularity.
These distortions extend to the oceans, which cover 71% of the surface. Without tides or currents, seawater would settle into a shape known as a geoid—a wavy, gravity-defined surface. Some areas rise where gravity is stronger, while others dip where it is weaker. These variations, called "geoid anomalies," reveal how mass is distributed deep within the Earth.
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