The Hyper-Kamiokande detector will be an underground radiation detector using the Cherenkov phenomenon, so it will record the charged particles produced by neutrinos and enable determination of their energies. It will be 5 times larger (258 kt water mass) than the current Super-Kamiokande detector. An extremely attractive feature of this type of detector is its simple operating principle: a large volume filled with very pure water, surrounded by light detectors (photomultipliers). The difficulty in the construction of Hyper-K will be the huge size of the tank – it will be a cylinder 71 m high and 68 m in diameter, which requires the hollowing out of a 73 m (H) by 69 m (D) cave in the rock. The whole construction must be placed deep underground (about 650 m), for shielding the detector from cosmic radiation. This size and location present new experimental and engineering challenges, as it will be the world’s largest man-made facility of its kind.