  It is quite plausible that the mass of the dark matter particle increases
significantly after its freeze-out, due to a scalar field rolling to large
values. We describe a realization of this scenario in the context of thermal
inflation which naturally gives a cold dark matter particle with the correct
cosmological abundance and a mass around $10^{10} {\rm GeV}$, evading the
conventional upper bound of $10^5 {\rm GeV}$. We also discuss another
realization which could produce a cosmologically interesting abundance of near
Planck mass, possibly electromagnetically charged, particles. The detection and
observational consequences of superheavy cold dark matter or WIMPZILLAs are
briefly examined.
