Borexino is an international experiment, located deep underground in the Gran Sasso National Laboratory of Italy. Its main goal is the study of the properties of a subatomic elementary particle, called the neutrino. The experiment is designed for the observation of low-energy (sub-MeV) solar neutrinos, with the specific goal of measuring the Be-7 neutrino flux from the Sun. 

In the pictures above: a view of the internal of the Borexino Stainless Steel Sfere and the inside view of Borexino detector filled with water.