  We present results from MMT/Hectochelle spectroscopy of red giant candidate
stars along the line of sight to the recently-discovered Galactic satellite
Crater 2. Modelling the joint distribution of stellar positions, velocities and
metallicities as a mixture of Crater 2 and Galactic foreground populations, we
identify 62 members of Crater 2, for which we resolve line-of-sight velocity
dispersion 2.7 +/- 0.3 km/s about mean velocity of 87.5 +/- 0.4 km/s. We also
resolve a metallicity dispersion 0.22 about a mean of [Fe/H]=-1.98 +/- 0.1 that
is 0.28 +/- 0.14 poorer than is estimated from photometry. Despite Crater 2's
relatively large size (projected halflight radius R(h)=1 kpc) and intermediate
luminosity (M_V =-8), its velocity dispersion is the coldest that has been
resolved for any dwarf galaxy. These properties make Crater 2 the most extreme
low-density outlier in dynamical as well as structural scaling relations among
the Milky Way's dwarf spheroidals. Even so, under assumptions of dynamical
equilibrium and negligible contamination by unresolved binary stars, the
observed velocity distribution implies a gravitationally dominant dark matter
halo, with dynamical mass 4.4 x 10^6 Msun and mass-to-light ratio M/L=53
enclosed within a radius of 1 kpc, where the equivalent circular velocity is
4.3 km/s.
