Originally Posted by pfdskipper
I believe you are referring to a witches eye. Those are circular little depressions that suddenly appear, seemingly randomly in the high dunes. They are also in the lower dunes as well although less frequently found,,, if I recall correctly.
They will ruin your day if you ride straight into one.
Hilslamers overview was pretty spot on as I recall.
I'm still running head on into these as I gain more experience in the dunes here and I've lost count of the number of really bad bruises across the tops of my thighs where I've struck the bars on my way over the front. Since the sand on the down slope into an eye is so soft I usually go from all ahead full to a dead stop in the blink of an eye. This always makes for a funny Go-Pro vid that I promptly delete!
An I agree, Hilslamer's overview was excellent and has proven very helpful.