  Little is known about the properties of the accretion flows and jets of the
lowest-luminosity quiescent black holes. We report new, strictly simultaneous
radio and X-ray observations of the nearby stellar-mass black hole X-ray binary
GS 2000+25 in its quiescent state. In deep Chandra observations we detect the
system at a faint X-ray luminosity of $L_X = 1.1^{+1.0}_{-0.7} \times
10^{30}\,(d/2 {\rm \,\, kpc})^2$ erg s$^{-1}$ (1-10 keV). This is the lowest
X-ray luminosity yet observed for a quiescent black hole X-ray binary,
corresponding to an Eddington ratio $L_X/L_{\rm Edd} \sim 10^{-9}$. In 15 hours
of observations with the Karl G. Jansky Very Large Array, no radio continuum
emission is detected to a $3\sigma$ limit of $< 2.8\ \mu$Jy at 6 GHz. Including
GS 2000+25, four quiescent stellar-mass black holes with $L_X < 10^{32}$ erg
s$^{-1}$ have deep simultaneous radio and X-ray observations and known
distances. These sources all have radio to X-ray luminosity ratios generally
consistent with, but slightly lower than, the low state radio/X-ray correlation
for stellar-mass black holes with $L_X > 10^{32}$ erg s$^{-1}$. Observations of
these sources tax the limits of our current X-ray and radio facilities, and new
routes to black hole discovery are needed to study the lowest-luminosity black
holes.

High Energy Astrophysical Phenomena