Polychlorinated biphenyls (PCBs) are compounds that were once used as insulators in electrical transmission lines and in the production of polymers. Each PCB differs by the quantity and location of the chlorine atoms. PCB production was halted in 1977 due to their potential toxicity, but the chemicals are still found in the environment due to their stability. Studies in remote areas of Alaska have shown that PCBs can even be found in lakes untouched by humans. There is no known natural process that produces PCBs, so all of the PCBs in existence are presumed to have been produced by humans. This interrupted case study is based on current research involving the global transport of PCBs. Students are asked to propose several hypotheses and experiments in an attempt to determine how PCBs are transferred globally. As the case unfolds, it becomes clear that the transport mechanism is more complicated than scientists first thought. The case requires minimal background knowledge and is suitable for major and nonmajor courses in biology, chemistry, and environmental science.