I believe it was under the surface, but I'm not sure because I didn't notice it until after I was finished cleaning the tank and had stirred up the sand a bit in the process. But right now there a spots of this blue-purple colored sand visible on the surface in spots and also under the surface visible from the side of the tank. I don't think it's anerobic pockets because every week I stir up the sand a bit so they don't have time to form. Also from what I have read, those pockets tend to be brown in color, not blue-purple.