  A Mn30 molecular cluster is established to be the largest single-molecule
magnet (SMM) discovered to date. Magnetization versus field measurements show
coercive fields of about 0.5 T at low temperatures. Magnetization decay
experiments reveal an Arrhenius behavior and temperature-independent relaxation
below 0.2 K diagnostic of quantum tunneling of magnetization through the
anisotropy barrier.The quantum hole digging method is used to establish
resonant quantum tunneling. These results demonstrate that large molecular
nanomagnets,having a volume of 15 nm^3, with dimensions approaching the
mesoscale can still exhibit the quantum physics of the microscale.
