I picked this one out of the archives as, to me, it is one of the most sci-fi looking constructs on the planet. Started in 1991, it's had a few major set backs but has still contributed to answering some of the big questions in physics.
Super-Kamiokande is a joint Japan-US collaboration to construct the world's largest underground neutrino observatory. This page is maintained by the US side of the collaboration. There is also an official Super-K home page maintained at the experiment site in Japan.
Neutrinos are subatomic particles which have no electrical charge, are very nearly massless, and interact only via the weak nuclear force. They are products of radioactive decay processes, and so are produced abundantly in our Sun, and in other astrophysical sources like supernovae and Active Galactic Nuclei (AGNs). Super-Kamiokande is located in the Kamioka Mine, about 200 km north of Tokyo, and is a water cerenkov detector , which means it is a large (40 meters diameter by 40 meters tall) tank of ultra-pure water viewed by thousands of sensitive phototubes. Super-Kamiokande will address some of the most important open questions in physics today, such as: why does the Sun appear to produce only half as many neutrinos as theory would predict? Do neutrinos have mass? Do protons decay, as predicted by Grand Unification Theory?