Is it circles or squares? Rotating it or viewing in the mirror shows something else! Based on math by Kokichi Sugihara. Best viewed at 45 degrees angle.
There are others like it here, but this one is designed with the original algorithms and math, which makes it as close to perfect shape as possible.
My other illusions: https://www.thingiverse.com/Matsemann/collections/matsemanns-ambiguous-cylinders
The code & stuff: https://github.com/Matsemann/impossible-objects