In 2019, Berlin-based artist Adam Harvey created a website called MegaPixels^[3] that flagged these and other data sets. He and another Berlin-based technologist and programmer, Jules LaPlace, showed that many had been shared openly and used to evaluate and improve commercial surveillance products. Some were cited, for instance, by companies that worked on military projects in China. “I wanted to uncover the uncomfortable truth that many of the photos people posted online have an afterlife as training data,” Harvey says. In total, he says he has charted 29 data sets, used in around 900 research projects. Researchers often use public Flickr images that were uploaded under copyright licences that allow liberal reuse.

After The Financial Times published an article on Harvey’s work^[4] in 2019, Microsoft and several universities took their data sets down. Most said at the time — and reiterated to Nature this month — that their projects had been completed or that researchers had requested that the data set be removed. Computer scientist Carlo Tomasi at Duke University was the sole researcher to apologize for a mistake. In a statement^[5] two months after the data set had been taken down, he said he had got institutional review board (IRB) approval for his recordings — which his team made to analyse the motion of objects in video, not for facial recognition. But the IRB guidance said he shouldn’t have recorded outdoors and shouldn’t have made the data available without password protection. Tomasi told Nature that he did make efforts to alert students by putting up posters to describe the project.

The removal of the data sets seems to have dampened their usage a little, Harvey says. But big online image collections such as MSCeleb are still distributed among researchers, who continue to cite them, and in some cases have re-uploaded them or data sets derived from them. Scientists sometimes stipulate that data sets should be used only for non-commercial research — but once they have been widely shared, it is impossible to stop companies from obtaining and using them.