  We address electrodynamic effects in plasma oscillations of a lossy 2D
electron system whose dc 2D conductivity is comparable to the speed of light.
We argue that the perpendicular constant magnetic field B causes astonishing
features of magnetoplasma dynamics. We show that plasmon-polariton spectra can
be classified using a 'relativistic' phase diagram 2D conductivity divided by
the speed of light versus B. An extraordinarily low damping branch in
magnetoplasmon-polariton spectra emerges at two phases of this diagram. Some
magnetoplasmons at these phases are predicted to be undamped waves.
