Can you see the 12 black spots outlined in white in this picture? All at once? It’s almost impossible to see them all at once on your first try. You can zip from one to another, but seeing them all at once will take some practice.
The 12-dots version of the illusion traces back to scientist and visual-illusions specialist Jacques Ninio, author of a paper on Hermann grid variations published in 2000 in the journal Perception. If the black-dots illusion isn't enough for you, then check out Ninio's reverse version in PDF format that has you searching for white dots against a black background. It's just as illusive.