Despite national and international curbs on their production, PCBs now congregate in the deep ocean, raising new concerns. In some areas of the Mariana, PCB levels registered 50 times higher than those found in crabs living in surface waters near heavy industry in China.

By the time the international community stepped in to end global PCB production, well over 1 million metric tons (about 3 billion pounds) had been manufactured worldwide. The 2001 UN Stockholm Convention on Persistent Organic Pollutants that resulted from these negotiations, but which the United States has yet to ratify, initially named DDT, PCBs, and 10 other chemicals (or classes of chemicals), all based on benzene.

“The challenge moving forward is to determine the physiological consequences of such contamination and understand knock-on effects on ecosystem function,” Jamieson and his colleagues concluded in the pages of Nature Ecology and Evolution. Except human activity may be altering the chemistry of the deep before we have had the chance to document it.

The problem is not limited to PCBs, but extends to the larger family of organohalogens to which they belong. In the early 1970s, PCBs were replaced by other organohalogens, one being polybrominated biphenyls, or PBBs, molecules similar to PCBs, but made with bromine instead of chlorine. Soon after their introduction, PBBs got into cattle feed, poisoning the food supply and the people of Michigan, and saddling the state with a long-lived legacy. PBDEs, polybrominated diphenyl ethers, followed as yet another alternative. These were used as flame retardants for two decades before a subsequent generation of Swedish scientists charted rising levels in breast milk.

Besides PCBs, amphipods living in the Mariana also harbor PBDEs, though the most prevalent commercial mixtures have been phased out of U.S. production and named to the UN Stockholm Convention as well.

The deep now archives nearly a century of chemical innovation, and documents the rise and fall of chemical classes, which industry develops and retracts in waves without seeming to absorb the larger lesson.

* * *

There are those who want a bright line to divide what’s natural from what isn’t as a means to make clear what’s safe. But with their origins in Earth’s deep carbon, and their enduring presence in life forms everywhere, such distinctions are murky at best. And yet, PCBs are part of a post-natural state in which industrial chemistry and ecology have become one and the same.

“Nature,” the organic chemist Pat Costner reminded me, “is a chemist, too” and the world its roiling, bubbling, reactive laboratory. Science has only begun to grapple with the complexity of our overlapping chemistries. Costner trained in organic chemistry at the peak of the chemical age. She took her first job as a bench chemist with Shell Oil in the 1960s, though she didn’t stay long. She turned her attention to organic pollution, particularly the chemical by-products industry never intended to make but released into nature anyway.