There are 12 black dots in the above picture, but one can’t see them all at the same time.
This was designed by Ninio & Stevens in 2000 and is pretty strong for me. It might look like it, but in my opinion the mechanism begin it is not a variation of the Hermann grid or Scintillating grid – notwithstanding the title of the paper cited below.
Tentative explanation: When one of the dot is fixated, its center and surround are well discerned. The next dots, however, top, left or whatever, fall on a region of our visual field that hast less resolution. There the black dot & white surround fuse, rendering the dot indistinguishable from the grid lines. [This cannot be the whole story.] Try out the following: Fixate one of the dots; then go in small steps towards the next one. Around half way there, it becomes visible.
Ninio J, Stevens KA (2000) Variations on the Hermann grid: an extinction illusion. Perception 29(10):1209–12017