Fig. 2: Dynamic compression of liquid water at 0.4 GPa/ms.

The sample pressure over time is measured using the SrB₄O₇ : Sm²⁺-chip fluorescence while micro-photographs are recorded at 40 kHz framing time. Errors bars are estimated to be  ±0.05 GPa following³⁰ and our system sensitivity. At the liquid-solid phase transition a characteristic pressure drop due the density difference is observed. Visual observation with the ultra-fast camera reveals a homogeneous nucleation in the sample volume. The over-compression pressure, ΔP, is defined as the pressure differential between crystallization and the associated ice VII melting at the same temperature (1.56 GPa). The quantity Δt represents the corresponding duration over which fluid water remains in its metastable state.