The stunning Super-Kamiokande is hidden beneath a mountain in Japan to detect neutrinos shot from supernovas.
To ensure that the cones of Cerenkov radiation successfully reach the Super K’s PMTs, the water inside the tank has to be super-pure. It’s continually re-purified and bombarded with UV light to kill off any bacteria floating around in it. The resulting liquid is so pure, it’s more like an acid and an alkaline than the H2O we know.
Moar stunning pictures of Super-Kamiokande.