There’s something weird going on with the Earth’s atmosphere. Scientists have found that a man-made gas banned long ago in a global agreement to save the ozone layer is mysteriously reappearing. But they’re not sure where it’s coming from.

Atmospheric measurements in Hawaii show that an ozone-depleting gas called CFC-11 is decreasing at a slower rate, which shouldn’t be the case if all the countries that signed on to the 1987 agreement are following the rules, a study published in the scientific journal Nature says.

“The evidence points to an increase in emissions nearly 10 years after the production was supposedly phased out, so it was very much a detective story for us to try to unravel,” said Stephen Montzka, a scientist at the National Oceanic and Atmospheric Administration and one of the study’s authors, told VICE News.

In 1987, 197 countries signed the Montreal Protocol, which set goals for decreasing the production of ozone-depleting chemicals like chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). A complete ban on the production of new CFCs began in 2010. Any new CFC production has to be reported to the United Nations Environment Programme. And the agreement has worked pretty well. A January NASA study found that there was 20 percent less ozone lost during the 2016 Antarctic winter than there was in 2005.

The ozone layer absorbs most of the sun’s UV rays and keeps them from reaching Earth. A change in the amount of UV rays making it through the atmosphere would be really bad for humans, plants, animals — anything that’s alive. UV radiation damages DNA, and can cause skin cancer and cataracts in people. The State Department says full implementation of the Montreal Protocol could prevent as many as 280 million cases of skin cancer in the U.S. alone by the end of the century.

CFC-11 can mostly be found in foam that was used to insulate houses and refrigerators produced before the 1990s. Montzka said that a small amount of CFC-11 will always be escaping as old refrigerators sit in landfills or insulation in old houses is destroyed.

“The CFC-11 in that foam is escaping to the atmosphere, so there is a residual emission of CFC-11, but we expect that emission to get smaller and smaller each year,” Montzka said.

And while CFC-11 concentrations in the atmosphere are decreasing overall, this study found the decrease to be slowing; CFC-11 concentrations decreased only about half as fast over the last three years as they did in the last decade. Beginning in 2013, atmospheric measurements in Hawaii started turning up larger concentrations of CFC-11.

The scientists looked at global wind patterns and the unique chemical signatures in the CFC-11–polluted air that made its way to Hawaii and concluded that the source of this newly released CFC-11 is somewhere in eastern Asia.

The specific location is a mystery, but Montzka said his colleagues from East Asia have long-term and location-specific emissions data that could help pinpoint exactly who is increasing the CFC-11 in the atmosphere.

A spokesman for the U.N. Environment Programme told The Washington Post that a panel of scientists would have to verify the study’s data before the U.N. could do anything. And there aren’t any punishments for violating the Montreal Protocol, so the bad actor would probably just negotiate a corrective action with the U.N.

If the U.N. doesn’t find a source, Montzka said, “It could delay ozone recovery by maybe a decade if it continued at the same rate.”

If the U.N. does find a source, the damage may not have a huge effect.

“If we are able to find the source of this emission and mitigate it, then I think that it's pretty clear that the damage in the ozone layer will be minor, if not insignificant,” Montzka said.