We observe the growth of crystalline SiC nanoparticles on Si(001) at 900 °C using in situ electron microscopy. Following nucleation and growth of the SiC, there is a massive migration of Si, forming a crystalline Si mound underneath each nanoparticle that lifts it 4-5 nm above the initial growth surface. The volume of the Si mounds is roughly five to seven times the volume of the SiC nanoparticles. We propose that relaxation of strain drives the mound formation. This new mechanism for relieving interfacial strain, which involves a dramatic restructuring of the substrate, is in striking contrast to the familiar scenario in which only the deposited material restructures to relieve strain.