Activation energies from shear viscosity and self-diffusion of highly supercooled liquid water rise very rapidly with decreasing temperature at 1 atm and approach approximate to 70 kJ/mol, or more, at 228 K. These high values may result because the flow of H2O molecules is impeded by passage through hydrogen-bonded pentagonal rings whose opening radius is approximate to 0.4 Angstrom, smaller than the van der Waals radius of H2O. Two principal effects may contribute to the overall energetics : (1) breakage of hydrogen bonds around a given H2O molecule; and then, only if no opportunity exists for free diffusion through hexagonal or larger rings in the immediate vicinity of the freed H2O molecule, (2) impeded, highly endothermic diffusion of that H2O molecule through a hydrogen-bonded pentagon, by forced ring expansion, followed by rebinding of the H2O. Large negative activation volumes are also indicated from 273 to 228 K and 1 atm. These negative volumes may arise from activated filling of voids in bulky, clathrate-like, polyhedral structures. A model for the diffusional process is described.