This exceptional #illusion was described 50 years ago by Celeste McCollough and it is still (1) an outstanding aftereffect that can last for months (!) and (2) not understood.
To celebrate these 50 years, I have now added it to my collection of visual phenomena and optical illusions https://michaelbach.de/ot/col-McCollough/
To help you make it through the boring minutes (!) of adaptation, I have added an interesting (?) fixation task.