WATER has been studied more extensively than any other liquid, Set its microscopic properties remain poorly understood. The difficulty in obtaining a rigorous molecular-scale description of water structure is largely a consequence of the extended, dynamic hydrogen-bonded network that exists throughout the liquid(1). Studies of the structure and dynamics of isolated small clusters of water molecules(2-6) provide a means of quantifying the intermolecular forces and hydrogen-bond rearrangements that occur in condensed phases. Experiments(2-7) and theory(8) strongly suggest that the water trimer, tetramer and pentamer have cyclic minimum energy structures. Larger water clusters are expected(8) to have three-dimensional geometries, with the hexamer representing the transition from cyclic to such three-dimensional structures. Here we report investigations by terahertz laser vibration-rotation tunnelling spectroscopy(3) of the structure of the water hexamer. A comparison of our results with quantum Monte Carlo simulations of this Species suggests that the most stable form of (H2O)(6) is indeed a cage-like structure, held together by eight hydrogen bonds (Fig. 1).