  The anharmonicity of atomic motion limits the thermal conductivity in
crystalline solids. However, a microscopic understanding of the mechanisms
active in strong thermal insulators is lacking. In this letter, we classify 465
experimentally known materials with respect to their anharmonicity and perform
fully anharmonic ab initio Green-Kubo calculations for 58 of them, finding 28
thermal insulators with $\kappa < 10$ W/mK including 6 with ultralow $\kappa
\lesssim 1$ W/mK. Our analysis reveals that the underlying strong anharmonic
dynamics is driven by the exploration of meta-stable intrinsic defect
geometries. This is at variance with the frequently applied perturbative
approach, in which the dynamics is assumed to evolve around a single stable
geometry.
