Despite being an everyday liquid, water is regarded as the most anomalous [1]: it expands upon cooling below 4 °C, or flows more easily when mildly pressurized. A hypothesis proposed 30 years ago [2] to explain these counter-intuitive behavior is still highly debated [1,3]. Water might exist in two liquid states, which would differ by the local arrangement of the molecules. However, the two liquids would separate only at very low temperature, and direct evidence for this transition remains elusive. Nevertheless, it is possible to gain insight about the origin of the anomalies of water even with experiments performed near room temperature, but at extreme pressures. The proposed doctoral work will cover extremely low (negative) and extremely high (GPa) pressures. The main tool used is Brillouin spectroscopy, an inelastic light scattering technique which gives access to the sound velocity and attenuation.