Super kamiokande detector, kamioka observatory, japan

"this 50′000 ton cylindrical ‘ring-imaging water cerenkov detector’ can be found at the kamioka mozumi mine in japan - 1′000m underground. clever people built the machine to detect neutrinos, proton decay and cosmic rays: this is done using the 12′000-ish photomultiplier tubes (extremely sensitive light detectors) visible on all walls of the ‘ultra-purified water-filled’ tank.

go here for a nicely simplified explanation and here for some incredible hi-res photos large enough to incapacitate the average pc.