The picturesque massif of the Apennines, Gran Sasso, is not only a paradise for skiers and nature lovers. It is also a very important place for physicists, chemists and data analysis specialists. For it was here, in the tunnel under the Gran Sasso d'Italia massif, that the particle physics laboratory was founded 35 years ago. Today, international teams carry out more than 20 advanced experiments there, requiring the best possible isolation from the background of cosmic radiation and natural radioactivity. Polish researchers from the Jagiellonian University are working together with scientists from Italy, France and Germany in one of such experiments - BOREXINO. The experiment allowed, among other things, to confirm the existence of an additional source of energy, the existence of which had been predicted for 80 years. Nuclear fusion on the Sun, to which we owe the energy, occurs by the fusion of hydrogen and helium. This fusion can take place in two ways: in the PP (proton-proton) cycle, where 99% of the energy is produced, but also in the CNO cycle, where the formation of helium is possible in the presence of other elements: carbon, nitrogen and oxygen. By measuring the fluxes of neutrinos from individual fusion reactions, the main mechanism of energy production in the Sun, the so-called PP cycle, which starts with the fusion of two protons, has been studied in detail. Now, with the first direct measurement of a relatively small neutrino flux from the CNO cycle, the BOREXINO team has proved the existence of this additional source of solar energy, and their findings have been published in Nature. "The first direct and accurate measurement of the flux of PP-type neutrinos from the fundamental thermonuclear reaction occurring in our nearest star was a huge success, while the registration of neutrinos from the CNO cycle is the culmination of our more than 25 years of research on solar neutrinos. It is an extraordinary intellectual adventure to participate in the confirmation of fundamental predictions related to the structure of stars," said Marcin Wójcik from the Neutrino Physics, Low Background Detectors, Dark Matter Search of the Jagiellonian University (after PAP Science in Poland).