These Nanobots Can Swim Around a Wound and Kill Bacteria

Most Americans Don’t Care If People Around Them Mask In Public, Poll Finds

Nanobots used for better root canal treatment

I'm trying to wrap my head around this picture taken by Curiosity on Mars a few days ago. What could've caused this doorway looking cutout on the side of that hill? None of the rocks laying in front of it are symmetrical enough to fit.