Neutrinos and antineutrinos detected in laboratories are normally generated by stars and nuclear reactors. An international team of physicists now has picked up neutrinos emanating from inside the earth. The antineutrino detector KamLAND, made up of nearly 2,000 light sensors placed around 1,000 tons of oil and fluorescent dye buried in a mine cavern in Japan, recorded roughly one antineutrino a month generated by the decay of uranium 238 and thorium 232 deep within the planet. The findings, in the July 28 Nature, suggest that the origin of the 30 trillion to 45 trillion watts of heat produced by the earth’s interior is about evenly divided between radioactive decay and leftover heat from the planet’s molten formation. Because the sound waves used to study the inner earth yield little about its chemistry, geoneutrinos should help shed light on the location and concentration of unstable elements.