Ultra-high energy cosmic rays present a number of puzzles involving mechanisms for particle acceleration, cosmology and physics
beyond the standard model. In particular a burning question is whether neutrinos are a substantial fraction of the flux around and above the
"GZK cutoff" at 10^20 eV. Several techniques are being developed to study such energetic neutrinos that rain onto the Earth with exceedingly small 
fluxes. Like in a good collider experiment the investigation of the unknown requires the use of different "sub-detectors" each measuring a 
particular aspect of the puzzle. In the last few years our group has pioneered the exploration of acoustic detection techniques, in which the 
neutrino-generated showers heat a large body of water producing an acoustic signal. Using a very large hydrophone array of the US Navy we have 
collected the first large volume sample of events and we are now in the process of instrumenting some 1500km^3 of sea water.