Decades after industrial waste dumping turned part of Southern California's seafloor into a toxic hot spot, scientists have dredged up a mystery. Chemicals fouling the ocean off the Palos Verdes Peninsula seem to be going away without being cleaned up. Samples taken from the sediment suggest more than 100 metric tons of the banned pesticide DDT and industrial compounds known as polychlorinated biphenyls, or PCBs, have vanished from one of the country's most hazardous sites, almost a 90% drop in just five years.