  Galaxy clusters are the most massive constituents of the large-scale
structure of the Universe. While the hot thermal gas that pervades galaxy
clusters is relatively well understood through observations with X-ray
satellites, our understanding of the non-thermal part of the intra-cluster
medium remains incomplete. With LOFAR and GMRT observations, we have identified
a phenomenon that can be unveiled only at extremely low radio-frequencies and
offers new insights into the non-thermal component. We propose that the
interplay between radio-emitting plasma and the perturbed intra-cluster medium
can gently re-energise relativistic particles initially injected by active
galactic nuclei. Sources powered through this mechanism can maintain electrons
at higher energies than radiative ageing would allow. If this mechanism is
common for aged plasma, a population of mildly relativistic electrons can be
accumulated inside galaxy clusters providing the seed population for
merger-induced re-acceleration mechanisms on larger scales such as turbulence
and shock waves.
