  The motion of a mechanical object -- even a human-sized object -- should be
governed by the rules of quantum mechanics. Coaxing them into a quantum state
is, however, difficult: the thermal environment effectively masks any quantum
signature of the object's motion. Indeed, it also masks effects of proposed
modifications of quantum mechanics at large mass scales. We prepare the
center-of-mass motion of a $10$ kg mechanical oscillator in a state with an
average phonon occupation of $10.8$. The reduction in oscillator temperature,
from room temperature to $77$ nK, represents a 100-fold improvement in the
reduction of temperature of a solid-state mechanical oscillator -- commensurate
with a 11 orders-of-magnitude suppression of quantum back-action by feedback --
and a 10 orders-of-magnitude increase in the mass of an object prepared close
to its motional ground state.
