  The central regions of galaxy clusters are permeated by magnetic fields and
filled with relativistic electrons. When clusters merge, the magnetic fields
are amplified and relativistic electrons are re-accelerated by turbulence in
the intra cluster medium. These electrons reach energies of 1 -- 10 GeV and, in
the presence of magnetic fields, produce diffuse radio halos that typically
cover an area of ~1 square Mpc. Here we report observations of four clusters
whose radio halos are embedded in much more extended, diffuse radio emission,
filling a volume 30 times larger than that of radio halos. The emissivity in
these larger features is about 20 times lower than the emissivity in radio
halos. We conclude that relativistic electrons and magnetic fields extend far
beyond radio halos, and that the physical conditions in the outer regions of
the clusters are quite different from those in the radio halos.
