You need to use a Java enabled browser to see this applet. There are several interesting things happening in this illusion . It always seems to happen wherever you are not looking. This proves your eye is round and can't focus all at once. All the spokes are identical, evenly placed and flat, but there is a definite feeling of depth throughout.
Some parts seem higher or lower than others.
Follow the most prominent ring of curves around, and watch the topography go from raised furrow to hollow trench. Read a short review of this book.