  Powerful quasars and radio galaxies are injecting large amounts of energy in
the form of radio plasma into the inter-galactic medium (IGM). Once this
nonthermal component of the IGM has radiatively cooled the remaining radio
emission is difficult to detect. Two scenarios in which the fossil radio plasma
can be detected and thus be used to probe the IGM are discussed: a)
re-illumination of the radio emission due to the compression in large-scale
shock waves, and b) inverse Compton scattered radiation of the cosmic microwave
background (CMB), cosmic radio background (CRB), and the internal very low
frequency synchrotron emission of the still relativistic low energy electron
population. We present 3-D magneto-hydrodynamical simulations of scenario a)
and compare them to existing observations. Finally, we discuss the feasibility
of the detection of process b) with upcoming instruments.
