An important scientific and technological goal(1-4) in the held of optical communications is the achievement of the clarity limit in optical fibres-that is, ensuring that the SiO2 glass from which fibres are made is as transparent as possible. The clarity of the wavelength transmission window (and the width of that window) in existing fibres is already sufficient to form the basis of a world-wide optical communication system(3,4), yet it is still limited by contamination of the fibre by water. Here we measure the spatial distribution of water in the glass rods from which optical fibres are drawn and explain the distribution quantitatively with a mathematical model of diffusion(5,6) in a medium with essentially perfect cylindrical symmetry. Our analysis shows that the water enters from the outside of the rod and diffuses into the molten, flowing glass much faster than is expected from extrapolation of low-temperature measurements(6,7). Our elucidation of the physics underlying the contamination process has already led to the fabrication of dry fibres(8,9), which have a clarified and broadened communications window. The improved operational range of wavelengths should yield applications for new lasers, optical amplifiers and detectors, and should substantially increase the information-carrying capacity of optical communications systems.