One of the most intriguing problems in science is and has always been the understanding of our Sun. Despite their elusiveness, millions upon millions of neutrinos hit each human being every second. Unless a neutrino scores a direct hit on an atomic nucleus (which only rarely occurs), it passes through without leaving a hint of its passage. Yet neutrinos are amongst the most important particles in the Universe. Because they emerge unscathed from the core of the Sun, they carry information about solar processes that we cannot observe otherwise. Approximately 97% of the energy released by the Sun is produced in the form of charged particles or photons. The remaining ~3% appears in the form of the kinetic energy of neutrinos. Ever since Davis and his collaborators started to observe the solar neutrino flux in the Homestake mine, we have for almost 30 years been faced with the problem of observing only about 30% of the expected neutrino flux. The solar neutrino problem originates from the discrepancy between the expected solar neutrino flux, as calculated by the standard solar model (SSM), and experimental results.