Mysterious, thermally insulating patches at the base of Earth's mantle

With modern seismic tomography, Earth scientists have discovered that above Earth's core-mantle boundary (CMB), about 2,900 kilometers beneath our feet, there is a thin layer about 300 kilometers thick with remarkable structural complexity and compositional heterogeneity. Among these features are small-scale structures known as ultralow velocity zones (ULVZs) that have attracted intense scientific interest.