This thesis presents experimental x-ray scattering studies on supercooled liquid water at near atmospheric pressure. Small-angle x-ray scattering (SAXS) experiments were performed at the X-ray lasers PAL-XFEL in Korea and SACLA in Japan, probing micrometer-sized water droplets down to temperatures of ≈227 K. In total, eight temperature-dependent maxima have been measured, namely: Intensity of the structure factor at small momentum transfer, temperature derivative of the first diffraction peak, isothermal compressibility and correlation length for both, H2O and D2O. The maxima occurred at temperatures of 229 K and 233 K for H2O and D2O, respectively. These results prove the existence of a Widom line in deeply supercooled water, which in turn explains the anomalous increase in waters thermodynamic properties that is observed upon cooling. The existence of the Widom line strongly points to the existence of a liquid-liquid critical point in supercooled water.