  We consider the detector size, location, depth, background, and radio-purity
required of a mid-Pacific deep-ocean instrument to accomplish the twin goals of
making a definitive measurement of the electron anti-neutrino flux due to
uranium and thorium decays from Earth's mantle and core, and of testing the
hypothesis for a natural nuclear reactor at the core of Earth. We take the
experience with the KamLAND detector in Japan as our baseline for sensitivity
and background estimates. We conclude that an instrument adequate to accomplish
these tasks should have an exposure of at least 10 kilotonne-years (kT-y),
should be placed at least at 4 km depth, may be located close to the Hawaiian
Islands (no significant background from them), and should aim for KamLAND
radio-purity levels, except for radon where it should be improved by a factor
of at least 100. With an exposure of 10 kT-y we should achieve a 24%
measurement of the U/Th content of the mantle plus core. Exposure at multiple
ocean locations for testing lateral heterogeneity is possible.
