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.
If you change the background luminance (right slider), the dots all become visible.
Ninio J, Stevens KA (2000) Variations on the Hermann grid: an extinction illusion. Perception 29(10):1209–12017