The Sudbury Neutrino Observatory (SNO) in Canada was officially opened this week. SNO will be the first detector capable of distinguishing between the three types of neutrino - electron, muon and tau neutrinos - and could finally resolve if these mysterious particles have mass or not. This is one of the outstanding questions in physics. The observatory, a joint project between Canada, US and UK, is a 1000-tonne tank of ultrapure heavy water located 2000 meters underground in a nickel mine in Ontario.