The purpose of this mega structure is a Neutrino Observatory to assist physicists study solar neutrinos, atmospheric neutrinos and search proton decay and detect neutrinos from a supernova some in our galaxy, to understand how matter was created at the beginning of the universe.
Light from particles travels through the water of the tank and interacst with the electrons/nuclei of water which produce particles that move faster than the speed of light (except if performed in a speed of light vacuum).
The movement of the particles are detected by the 11,000 photo-multiplier tubes on the walls, the distinct pattern of this flash provides information on the direction and information of the incoming neutrino. From here, the physicists are able to analyse the particle properties on their computers. The process creates a flash of light which is the optical equivalent to a sonic boom.