  Direct detection of gravitational radiation in the audio band is being
pursued with a network of kilometer-scale interferometers (LIGO, Virgo, KAGRA).
Several space missions (LISA, DECIGO, BBO) have been proposed to search for
sub-Hz radiation from massive astrophysical sources. Here we examine the
potential sensitivity of three ground-based detector concepts aimed at
radiation in the 0.1 -- 10\,Hz band. We describe the plethora of potential
astrophysical sources in this band and make estimates for their event rates and
thereby, the sensitivity requirements for these detectors. The scientific
payoff from measuring astrophysical gravitational waves in this frequency band
is great. Although we find no fundamental limits to the detector sensitivity in
this band, the remaining technical limits will be extremely challenging to
overcome.
