In 1987, after tons and tons of ozone-depleting substances had grown a hole in the layer of gas that serves as a big slather of sunscreen for the whole planet, a massive intergovernmental agreement, the Montreal Protocol, was put in place to stem the flow of chlorofluorocarbons. This family of chemicals was heavily used in aerosols, refrigerators, air conditioners and others up until they were banned that year.

The program was heralded as a huge political success—a rare bout of international environmental cooperation. And it worked. Or, we thought it worked.

Not long after the Montreal Protocol, countries' self-reported emissions rates of ozone-depleting substances plummeted. Yet according to new research, the atmospheric concentration of one important ozone-depleting substance—carbon tetrachloride—isn't dropping anywhere near as fast as it should be. Researchers suspect that someone—they don't know who, or where, or why—is still emitting carbon tetrachloride into the atmosphere. The emissions come to the tune of 39,000 tons per year, roughly 30 percent of what carbon tetrachloride emissions were before the ban.

“This very large emissions estimate difference is equivalent to ~1600 railroad tank cars of liquid [carbon tetrachloride],” the researchers write in their study.

That's a lot of gas to be drifting away under everyone's noses.