Metal nanoparticles supported by thin films are important in the fields of molecular electronics, biotechnology and catalysis, among others. Penetration of these nanoparticles through their supporting films can be undesirable in some circumstances(1) but desirable in others(2-4), and is often considered to be a diffusive process. Here, we demonstrate a mechanism for the penetration of thin films and other nanoscopic barriers that is different from simple diffusion. Silver clusters that are soft-landed onto a monolayer of C-60 supported by gold sink through the monolayer in a matter of hours. However, the clusters are stable when landed onto two monolayers of C-60 supported on gold, or on one monolayer of C-60 supported on graphite. With backing from atomistic calculations, these results demonstrate that a metallic substrate exerts attractive forces on metallic nanoparticles that are separated from the substrate by a single monolayer.