There’s a gray area in artificial intelligence filled with millions of humans who work in secret — they’re often hired to train algorithms but end up operating much of their work instead.

These crucial workers took the spotlight this week when The Information reported that Amazon’s Just Walk Out technology, which allowed customers to grab grocery items from a shelf and walk out of the store, was being phased out of its grocery stores. It partially relied on more than 1,000 people in India who were watching and labeling videos to make sure the checkouts were accurate.

Amazon says on its website that Just Walk Out uses "computer vision, sensor fusion and deep learning” but doesn’t mention contractors. The company told Gizmodo that the workers were annotating videos to help improve them and that they validated a "small minority” of shopping visits when its AI couldn’t determine a purchase.