Researchers have figured out how make shaped drone rotors that convert audible and ultrasound frequencies into stable mechanical spinning motion, leading to tiny drones that can be completely silent. While scary-sounding (no pun intended) it’s unclear how usable these would be in the real world, since a line-of-sight source of sound waves has to be provided.
It looks like grey goo and skynet are competing to see who will be the next apocalypse.