Water under pressure should slow down. In most liquids, compressing the molecules together makes structural rearrangement harder — the relaxation time increases monotonically with pressure. Water does the opposite.
Ranieri et al. (arXiv:2603.17793) measure both shear and bulk viscosity of water simultaneously up to 1.6 GPa using light scattering. The shear viscosity increases with pressure, as expected. But the structural relaxation time — the timescale on which hydrogen bond networks rearrange between local configurations — first decreases, reaching a minimum of about 1 picosecond near 0.5 GPa, before eventually increasing at higher pressures.
At ambient pressure, water's hydrogen bond network is open and tetrahedral. Compression initially disrupts this open structure, allowing molecules to interconvert between local configurations more rapidly. The network becomes less ordered but more dynamic. Only at much higher pressures does the simple crowding effect take over, slowing everything down as in ordinary liquids.
The minimum relaxation time marks the pressure at which water transitions from anomalous to normal liquid behavior. Below this pressure, compression speeds things up (anomalous). Above it, compression slows things down (normal). The ratio of shear to bulk viscosity drops by half across the pressure range, reflecting the changing balance between flow and structural rearrangement.
The authors propose that this single relaxation minimum generates a cascade of thermodynamic and dynamic anomalies — the density maximum, the compressibility minimum, the diffusion anomaly. Water is strange not because it has many independent anomalies, but because it has one structural feature — a pressure where rearrangement is fastest — and the anomalies are consequences of that feature propagating through different observables.