  There are still several unanswered fundamental questions concerning our
planet and in particular, about the deep Earth, from where we lack direct
samples. Today, due to the progress in neutrino-detection techniques, a new and
unique tool to study our planet exists: geoneutrinos, antineutrinos from the
decays of long-lived radioactive elements inside the Earth. In 2020, the
Borexino experiment published its updated geoneutrino measurement. Thanks to
the increase in acquired data and the improved analysis techniques in an
enlarged fiducial volume, the final precision has significantly improved,
corresponding to a total geoneutrino signal of 47.0$^{+8.4}_{-7.7}\,({\rm
stat)}^{+2.4}_{-1.9}\,({\rm sys})$ TNU. The null-hypothesis of the geoneutrino
signal from the Earth's mantle was excluded at a 99% C.L. for the first time,
while exploiting the knowledge of the local crust around the detector. The
article will introduce the field and describe the key elements of the updated
analysis. Geological interpretation and significance of the new result will be
discussed in terms of the corresponding radiogenic heat and the limits to a
hypothetical georeactor.
