The 1987 Montreal Protocol is widely touted as an environmental success story, one that staved off expansion of the ozone hole above Antarctica (pictured). But someone is cheating on that international agreement, The Washington Post reports. Scientists have detected a 25% uptick in emissions of CFC-11—an ozone-destroying chemical—since 2012. CFC-11 was once commonly used in insulating foams, but it’s now banned under the Montreal Protocol and reported production is close to zero. The startling resurgence of the chemical, reported in Nature, will likely spark an international investigation to track down the mysterious source.