Fresh experimental and theoretical results on thermally induced catastrophic breakdown (the fiber fuse) in optical fibers are presented, including the observation that the damage is not always irreversible and an analysis of the complex unsteady absorption-heat-conduction process that controls the effect. Good agreement with experiment is obtained with just two independent parameters. The analysis shows that the fiber fuse is a new kind of solitary thermal shock wave in whose leading edge the temperature gradients can reach several thousand kelvins per micrometer.