A UC Berkeley computer science professor helped to create a video that imagined a world where nuclear weapons were replaced by swarms of autonomous tiny drones that could kill half a city and are virtually unstoppable.

Stuart Russell, the professor, said these drones are already a reality.

The video takes the viewer to an auditorium where a speaker showcases a drone roughly the size of a mockingbird. At one point the drone lands on his hand, the speaker quickly recalibrates it and then throws it out into the audience again. After a few seconds, the small drone turns back to the stage and crashes into the forehead of a dummy standing off to the left of the speaker.

The demonstration was meant to show how a palm-sized drone is capable of penetrating a human’s skull and destroying “the contents" inside.

The video was released earlier this month by the Future of Life Institute, which is backed by Stephen Hawking and Elon Musk. It was presented by Russell at a United Nations Convention on Conventional Weapons in Geneva, according to The Mercury News.

“Trained as a team, [the drones] can penetrate buildings, cars, trains, all while having the capacity to evade any countermeasure. They cannot be stopped,” the speaker told the audience in the video.

He noted that “a $25 million order” can now buy a swarm of such tiny “slaughterbots” that could kill half a city.

Russell said that although A.I.’s “potential to benefit humanity is enormous, even in defense,” allowing the widespread use of machines that “choose to kill humans will be devastating to our security and freedom.”

“Thousands of my fellow researchers agree. But the window to act is closing fast,” he said.