  Despite 100 years of effort, we still know very little about the origin of
ultra-high energy cosmic rays. The observation of neutrinos produced when
cosmic-ray protons with energies above $4\times 10^{19}$ eV interact with the
cosmic microwave background radiation, or in the neutrino sources, would tell
us much about the origin and composition of these particles. Over the past
decade, many experiments have searched for radio waves emitted from the charged
particle showers produced when EHE neutrinos interact with Antarctic or
Greenland ice or the moon. These experiments have not yet observed a neutrino
signal. Two groups are now proposing to instrument 100 km$^3$ of Antarctic ice
with radio antennas, producing a detector large enough to observe a clear EHE
neutrino signal in a few years of operation.
