  A new model for the, so called, `cluster radio relics' is presented (Ensslin
& Gopal-Krishna 2000). Fossil radio cocoons, resulting from the former activity
of radio galaxies, should contain a low energy relativistic electron population
and magnetic fields. Electrons with an age of even up to 2 Gyr can be
re-accelerated adiabatically to radio emitting energies, if the fossil radio
plasma gets compressed in an environmental shock wave. Such a wave can be
caused by a merging event of galaxy clusters, or by the accretion onto galaxy
clusters. An implication of this model is the existence of a population of
diffuse, ultra-steep spectrum, very low-frequency radio sources located inside
and possibly outside of clusters of galaxies, tracing the revival of aged
fossil radio plasma by the shock waves associated with large-scale structure
formation.
