I have a beautiful 5 year old bathroom . The floor is marble and the walls are the same in the shower and adjacent little towel closet over brick and cement walls and cement floors . I noticed since one year that sand accumulates in one corner of the shower ...on top of the marble floor ....just in one corner ...a small little mountain about 6 inches by 8 inches . And , I noticed in the adjacent closet that there is a much larger built up of just sand inside the closet where the walls are only made off cement covered brick ! At first I thought perhaps ants got into the wall ....there is no wood , only cement ... underneath the marble floor and behind the marble walls is also cement ... the entire construction of the house is bricks and cement ...with a cement floor ...and there are NO ants . I looked with a microscope ...ANYONE has any idea what this could be ?????????? PLEASE , !