Low concentrations of three distinct impurity species beneath the Pd(111) surface are studied by STM. The subsurface impurities are distinguished by their image contrast, diffusion properties, and interactions with adsorbed molecules. Isolated subsurface impurities appear at low gap resistance (less than or similar to5 M Omega) as three-fold symmetric modulations of the Pd 1x1 surface corrugation. One impurity type is found to occupy substitutional sites in the layer below the surface. Based on Auger spectroscopy this species is identified as sulfur. The other two species are found to occupy octahedral interstitial sites immediately below the surface layer. Two-dimensional diffusion of the interstitial impurities occurs below room temperature. The onset temperature for diffusion is lowered dramatically in the presence of surface adsorbates. Quantitative measures of the diffusion barriers are consistent with surface facilitated diffusion of interstitial oxygen and carbon atoms. The mobile impurities interact with adsorbed atoms and molecules, limiting surface diffusion, nucleating island growth, and serving as active sites for surface reactions.