A density grating buried under a smooth surface is reported for an azobenzene-containing polymer film. The grating is formed by annealing a polymer sample pre-inscribed with a surface grating for several hours above T-g, creating a lateral density difference on the order of 10 %. It is shown that the rate of the process can be influenced by the intensity and polarization of the probe laser. Growth of photoinduced liquid-crystalline aggregates is proposed as the most probable explanation for the observed behavior.