  The deepest hole that has ever been dug is about 12 km deep. Geochemists
analyze samples from the Earth's crust and from the top of the mantle.
Seismology can reconstruct the density profile throughout all Earth, but not
its composition. In this respect, our planet is mainly unexplored.
Geo-neutrinos, the antineutrinos from the progenies of U, Th and K40 decays in
the Earth, bring to the surface information from the whole planet, concerning
its content of natural radioactive elements. Their detection can shed light on
the sources of the terrestrial heat flow, on the present composition, and on
the origins of the Earth. Geo-neutrinos represent a new probe of our planet,
which can be exploited as a consequence of two fundamental advances that
occurred in the last few years: the development of extremely low background
neutrino detectors and the progress on understanding neutrino propagation. We
review the status and the prospects of the field.
