  Condensed matter in the low temperature limit reveals much exotic physics
associated with unusual orders and excitations, examples ranging from helium
superfluidity to magnetic monopoles in spin ice. The far-from-equilibrium
physics of such low temperature states may be even more exotic, yet to access
it in the laboratory remains a challenge. Here we demonstrate a simple and
robust technique, the 'magnetothermal avalanche quench', and its use in the
controlled creation of nonequilibrium populations of magnetic monopoles in spin
ice at millikelvin temperatures. These populations are found to exhibit
spontaneous dynamical effects that typify far-from-equilibrium systems, yet are
captured by simple models. Our method thus opens the door to the study of
far-from-equilibrium states in spin ice and other exotic magnets.
