  When high-energy particles interact in dense media to produce a particle
shower, most of the shower energy is deposited in the medium as heat. This
causes the medium to expand locally and emit a shock wave with a
medium-dependent peak frequency on the order of 10 kHz. In South Pole ice in
particular, the elastic properties of the medium have been theorized to provide
good coupling of particle energy to acoustic energy. The acoustic attenuation
length has been theorized to be several km, which could enable a sparsely
instrumented large-volume detector to search for rare signals from high-energy
astrophysical neutrinos. We simulated a hybrid optical/radio/acoustic extension
to the IceCube array, specifically intended to detect cosmogenic (GZK)
neutrinos with multiple methods simultaneously in order to achieve high
confidence in a discovered signal and to measure angular, temporal, and
spectral distributions of GZK neutrinos.
  This work motivated the design, deployment, and operation of the South Pole
Acoustic Test Setup (SPATS). The main purpose of SPATS is to measure the
acoustic attenuation length, sound speed profile, noise floor, and transient
noise sources \emph{in situ} at the South Pole. We describe the design,
performance, and results from SPATS. We measured the sound speed in the fully
dense ice between 200 m and 500 m depth to be 3878 $\pm$ 12 m/s for pressure
waves and 1975.8 $\pm$ 8.0 m/s for shear waves. We measured the acoustic
amplitude attenuation length to be 316 $\pm$ 105 m. We measured the background
noise floor to be Gaussian and very stable on all time scales from one second
to two years. Finally, we have detected an interesting set of
well-reconstructed transient events in over one year of high quality transient
data acquisition. We conclude with a discussion of what is next for SPATS and
of the prospects for acoustic neutrino detection in ice.
