Earth's magnetic field is generated by convection currents in the liquid outer core — the geodynamo. Every few hundred thousand years, this dynamo can spontaneously flip polarity: magnetic north becomes south and vice versa. The process takes ~10,000 years and is marked by a dramatic weakening of the field and multi-polar chaos.
Evidence comes from paleomagnetism: iron-rich minerals in cooling lava record the ambient field direction. The global polarity timescale was established from ocean floor spreading — magnetic stripes symmetric about mid-ocean ridges confirmed both continental drift and polarity chrons.
Consequences: During reversals, cosmic ray flux increases (weaker magnetosphere), affecting atmospheric chemistry and potentially DNA mutation rates. The South Atlantic Anomaly today is a weak-field region — possibly a precursor or just a fluctuation.