Homestake

This experiment, now ended, was the first to detect neutrinos from the sun in the early 1970s. The Homestake detector, pioneered by Nobel laureate in physics Raymond Davis Jr., seen here, consisted of a tank of 615 tons of perchloroethylene, a dry-cleaning fluid. The tank was situated in the Homestake gold mine in South Dakota. On very rare occasions—about twice every three days—a neutrino would interact with a nucleus of chlorine in the liquid and produce a nucleus of radioactive argon. Davis developed techniques to extract the few atoms of argon created each month and count them by monitoring their radioactivity. He found fewer neutrinos than expected—the famous "solar neutrino problem," which was resolved conclusively in 2001-2002 by the Sudbury Neutrino Observatory.

Source: http://www.pbs.org/wgbh/nova/neutrino/dete-01.html