An accidental optical illusion

#1 My perception of this picture swings between the structure in the centre of the picture being a protuberance and a depression. This happens to me even though I saw it in real life, and know which parts are supposed to be deeper than others.

Beside it cheating my eye, the structure in itself is also interesting since it resembles a fractal tree. Nature is awesome.

The structure is a sand formation at the beach down by the surf. So it is made by water streaming downward, from the top of the picture toward the bottom in the direction of the sea.

#2 Here if I look at the part of the structure that is in shadow (right) it looks like a depression, while the part in sun (left) looks like a protuberance

#3 For reference here is the structure where it is actually possible to perceive depth correctly (at least for me).

I know other people can see it one or the other way, and it cheats someone into thinking it is a protuberance and not a depression.