We report scanning tunneling microscopy observations, which imply that all
atoms in a Cu(001) surface move frequently, even at room temperature. Using
a low density of embedded indium "tracer" atoms, we visualize the diffusiv
e motion of surface atoms. Surprisingly, the indium atoms seem to make conc
erted, long jumps. Responsible fur this, motion is an ultralow density of s
urface vacancies, diffusing rapidly within the surface. This interpretation
is supported by a detailed analysis of the displacement distribution of th
e indium atoms, which reveals a shape characteristic fur the vacancy mediat
ed diffusion mechanism that we propose.