It sneaks in through dusty border checkpoints in boxes marked "tomatoes." Its very existence is, according to many, a threat to the future health of the planet. And on the black market, it can turn a better profit for smugglers than cocaine.
It's chlorofluorocarbon-12, the greenhouse gas most often connected with the depletion of the ozone layer. And despite the efforts of the United States - and most of the world community - to stop its use, it remains one of the most intractable problems facing US Customs today.
When the manufacture and importation of the gas, known as CFC12, was banned throughout much of the world in 1996, its use in the US was expected to stop when stocks ran out.
A nascent black market exploded, feeding CFC12 to auto-repair shops and refrigeration companies, which used it in older-model cooling systems. For a few years prior to its complete ban, huge import taxes made it enormously expensive. Rather than paying high prices or replacing old cooling systems with ones that use more ozone-friendly compounds, some in the auto and refrigeration industries turned to the illegal market, creating problems for US interdiction officials.
Customs officers along the California-Mexico border have seized both large and small amounts of smuggled CFCs, still produced legally in places like China and Eastern Europe.
"You name it, they've tried it," says Marc Gwaltney, a US Customs official in Los Angeles. He cites a taco stand in Tijuana, Mexico. "In addition to a taco, you could buy a one-pound can of Freon [a brand of CFC], put it in your pocket, and walk back across the border."
Still, the Customs Service, Environmental Protection Agency, and Justice Department have scored notable successes.
In Miami, for example, operation Cool Breeze, an interagency task force, seized more than 1 million pounds of contraband CFCs, leading to 17 convictions and the return of $5.4 million from overseas accounts linked to smuggling operations.
"We've taught them if they want to smuggle CFCs to try it someplace else," says Keith Prager, a Miami Customs agent.
BUT even as stringent controls at major American ports and border crossings have forced CFC smugglers to shift their focus to the rural Texas-Mexico border, the Canadian border, and third-world markets, federal scientists monitoring the ozone layer are still worried.
Several months ago, the ozone hole over the Antarctic expanded to an area roughly the size of North America - the biggest hole ever recorded. And while the use of chlorofluorocarbons (with the exception of CFC12) is declining worldwide, other developments are causing concern.
Recent studies show the greenhouse warming phenomenon appears to be part of a previously unknown "feedback loop." The loop cools the upper atmosphere - thus contributing significantly to the ozone layer's degradation.
"We're worried that all the improvements we've made in limiting these chemicals may be inhibited by this global warming," says Jim Elkins, at the National Oceanic and Atmospheric Administration in Boulder, Colo. "We're holding our own, but this feedback mechanism could prolong the time necessary to restore the ozone layer 10, 20 years or longer."