  A study of fossil tracks of charged particles recorded in crystals of
muscovite has revealed evidence of rare events of cosmological origin. The
events are not compatible with known particle interactions with matter. They
were recorded during a period when the crystals were in a metastable state
during cooling after growth 13km water equivalent underground. In this state a
phase transition can be triggered by low energy events in the range 1eV to
10keV, when the crystals effectively behave as solid-state bubble chambers. At
higher energies the chemical etching technique can be used to reveal massive
damage to the lattice. The rare events show evidence of interaction with the
crystal over a great range of energies. They leave a distinctive record that is
easily recognised.
