China’s Three Gorges Dam is so massive that scientists have calculated it caused a minute change in Earth’s rotation. The effect is measured in fractions of a microsecond per day — far too small for humans to notice. The phenomenon occurs because the dam holds back an enormous volume of water. When that mass is redistributed farther from Earth’s rotational axis, it slightly increases the planet’s moment of inertia, similar to how a spinning figure skater slows down when extending their arms. The