  The detection of electron anti-neutrinos from natural radioactivity in the
earth has been a goal of neutrino researchers for about half a century. It was
accomplished by the KamLAND Collaboration in 2005, and opens the way towards
studies of the Earth's radioactive content, with very important implications
for geology. New detectors are operating (KamLAND and Borexino), building
(SNO+) and being proposed (Hanohano, LENA, Earth and others) that will go
beyond the initial observation and allow interesting geophysical and
geochemical research, in a means not otherwise possible. Herein we describe the
approaches being taken (large liquid scintillation instruments), the
experimental and technical challenges (optical detectors, directionality), and
prospects for growth of this field. There is related spinoff in particle
physics (neutrino oscillations and hierarchy determination), astrophysics
(solar neutrinos, supernovae, exotica), and in the practical matter of remote
monitoring of nuclear reactors.
