An aperiodic monotile
A new mathematical discovery as of 2023, this is a single shape that can be repeated to tile the plane (a monotile), but it tiles the plane in such a way that the pattern it creates never repeats (aperiodic). Apparently, no one knew of a shape that had this property before 2023. It's a cool thing about math that "basic"-seeming things like this are still out there to be found!
