On 30 June this year, a study was released in one of the world's top scientific journals. It explained how a group of scientists who had been measuring the amount of ozone in the stratosphere had made a startling observation: the hole in the ozone layer had shrunk. Here, they said, was the first clear evidence that the ozone layer had begun to heal. So how did this happen?

Helena Merriman tells a story that involves dogged scientific endeavour, the burgeoning green movement of the 1980s and the signing of what has been described as the most successful treaty ever created.

Producers: Lucy Proctor and Hannah Sander.