Microscopic plants and animals congregate in the top millionth of a metre in any body of water. Toxic pollutants also accumulate in this microlayer, in concentrations many times those of the water below. Scientists theorize that pollution from the air settles on the surface, some components of sewage and industrial waste float to the surface, and pollutants which had previously fallen to the seabed rise up to the surface when the seabed is disturbed by currents or the movements of animals.