Deep in Earth’s crust, pressure exceeds 1000 times the atmospheric pressure. Water still flows under these conditions, but experiences dramatic changes in structure and fluidity. Using combined dynamic and inelastic light scattering techniques, we simultaneously measure the shear and bulk viscosities of water as a function of pressure. The former increases faster than the latter, so that their ratio shows a twofold decrease from 0 to 1.6 GPa; we confirm this trend with simulations. We analyze our results in terms of the structural relaxation time $\tau$. Contrary to other liquids, pressure initially accelerates relaxation in water. Our measurements reveal that $\tau$ reaches a minimum close to 1 ps around 0.5 GPa. We interpret $\tau$ as the equilibration time of hydrogen bonds and propose that the minimum in $\tau$ arises from a structural anomaly that allows fastest interconversion between local structures in water and generates a cascade of thermodynamic and dynamic anomalies.

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• Elastic modulus
• High-pressure studies
• Ultrasound attenuation
• Water
• Viscosity
• Brillouin scattering & spectroscopy
• Light scattering
• Molecular dynamics