Mantle minerals at shallow depths contain iron in the high-spin electronic state. The crystal-field splitting energy increases with increasing pressure, which can favor the low-spin state. Hence, pressure-driven transitions from the high-spin to the low-spin state were proposed as early as the 1960s, and minerals in the lower mantle were suggested to contain iron in the low-spin state. Only in the past 10 years did experiments and calculations prove that iron in mantle minerals transforms from high-spin to low-spin at lower-mantle pressures. This transition has important consequences for volume, thermodynamics, and bonding. In a geophysical framework, the transition would affect the dynamics and thermochemical state of the lower mantle, through combined effects on density, elasticity, element partitioning, and transport properties. These observations provide the basis for a new paradigm of the physics and chemistry in Earth's lower(most) mantle.