“When you’re out there, and you can hear them moving around, but you can’t see them, it’s like, ‘Where are they going to come from?’,” she said. “It’s a little nerve-racking at first.”

Amazon is increasingly requiring warehouse employees to get used to working with robots. The company now has more than 200,000 robotic vehicles it calls “drives” that are moving goods through its delivery-fulfillment centers around the U.S. That’s double the number it had last year and up from 15,000 units in 2014.

Its rivals have taken notice. Many are adding their own robots in a race to speed up productivity and bring down costs.