One thousand metres underground in a mine in Kamioka in Japan, a huge tank of water, 42 m high and 39 m in diameter, watches for the interactions of the elusive particles called neutrinos. The walls, ceiling and floor of the tank --- which is the major part of a detector called SuperKamiokande are covered at regular intervals by 11 146 light-sensitive phototubes, each about 50 cm in diameter. These pick up light (Cherenkov radiation) emitted as the energetic charged particles produced in the neutrino interactions travel through the water. This picture (courtesy of SuperKamiokande) shows about 9 000 of the tubes --- the small bright spots --- on the walls and ceiling of the tank, before it was filled with water. SuperKamiokande detects neutrinos emitted in nuclear interactions in the Sun, and also in the interactions of cosmic-ray particles in the atmosphere. Measurements of these "atmospheric neutrinos" suggest that neutrinos may "oscillate" --- change from one type to another --- which they can do only if they have some mass, although this mass must be very, very small.
See also:
Advances in nu Physics: nu Evidence for nu Masses
The Mathematics of Neutrino Oscillations