You know how I love my optical illusions so here is a nice one from the Mozilla developer demo site (cira 2015). When I saw it first I just saw a mess of concentric circles. Now whenever I look at it, I can only see the 3D shape. There's a few other demos on the current Mozilla demo site, but this page is about 3D Spirals. Here is the example recreated here.

Written by Chris Heilmann - @codepo8

Not quite spiral, move like tubes, but they still have the same effect on the viewer!

http://justtrance.net/