As impossible as it may seem, Fraser's Spiral (also known as the twisted cord illusion) is not a spiral but a series of concentric circles. Click on the reveal button to confirm this. The effect is created by adding an unaligned design (the barber's pole stripes) to a regular pattern (the concentric circles).
References
[1]
K. Fraser J, "A New Visual Illusion of Direction," British Journal of Psychology, vol. 2, pp. 307–320, 1908.