  We map the distribution and properties of the Milky Way's interstellar medium
as traced by diffuse interstellar bands (DIBs) detected in near-infrared
stellar spectra from the SDSS-III/APOGEE survey. Focusing exclusively on the
strongest DIB in the H-band, at ~1.527 microns, we present a projected map of
the DIB absorption field in the Galactic plane, using a set of about 60,000
sightlines that reach up to 15 kpc from the Sun and probe up to 30 magnitudes
of visual extinction. The strength of this DIB is linearly correlated with dust
reddening over three orders of magnitude in both DIB equivalent width (W_DIB)
and extinction, with a power law index of 1.01 +/- 0.01, a mean relationship of
W_DIB/A_V = 0.1 Angstrom mag^-1, and a dispersion of ~0.05 Angstrom mag^-1 at
extinctions characteristic of the Galactic midplane. These properties establish
this DIB as a powerful, independent probe of dust extinction over a wide range
of A_V values. The subset of about 14,000 robustly detected DIB features have
an exponential W_DIB distribution. We empirically determine the intrinsic rest
wavelength of this transition to be lambda_0 = 15,272.42 Angstrom, and then
calculate absolute radial velocities of the carrier, which display the
kinematical signature of the rotating Galactic disk. We probe the DIB carrier
distribution in three dimensions and show that it can be characterized by an
exponential disk model with a scaleheight of about 100 pc and a scalelength of
about 5 kpc. Finally, we show that the DIB distribution also traces large-scale
Galactic structures, including the central long bar and the warp of the outer
disk.
