  The study of the universe at energies above 100 GeV is a relatively new and
exciting field. The current generation of pointed instruments have detected TeV
gamma rays from at least 10 sources and the next generation of detectors
promises a large increase in sensitivity. We have also seen the development of
a new type of all-sky monitor in this energy regime based on water Cherenkov
technology (Milagro). To fully understand the universe at these extreme
energies requires a highly sensitive detector capable of continuously
monitoring the entire overhead sky. Such an instrument could observe prompt
emission from gamma-ray bursts and probe the limits of Lorentz invariance at
high energies. With sufficient sensitivity it could detect short transients
($\sim$15 minutes) from active galaxies and study the time structure of flares
at energies unattainable to space-based instruments. Unlike pointed instruments
a wide-field instrument can make an unbiased study of all active galaxies and
enable many multi-wavelength campaigns to study these objects. This paper
describes the design and performance of a next generation water Cherenkov
detector. To attain a low energy threshold and have high sensitivity the
detector should be located at high altitude ($>$ 4km) and have a large area
($\sim$40,000 m$^2$). Such an instrument could detect gamma ray bursts out to a
redshift of 1, observe flares from active galaxies as short as 15 minutes in
duration, and survey the overhead sky at a level of 50 mCrab in one year.
