Water is the most important liquid for life on Earth and plays an essential role in physics, chemistry, biology and geoscience. However, the microscopic origin of the anomalous properties of water, such as density maximum at 4 degree C, has been elusive and there has been an intense debate for over a century. One major hypothesis, that has strong indirect support from theoretical work, is so-called liquid-liquid critical point (LLCP) scenario. In this talk, the first direct experimental evidence of the existence of the Widom line which is supposed to emanate from the LLCP will be presented (1). The experiment was performed at beamline NCI of PAL-XFEL. Reference (1.) K. H. Kim, A. Späh, H. Pathak, F. Perakis, D. Mariedahl, K. Amann-Winkel, J. A. Sellberg, J. H. Lee, S. Kim, J. Park, K. H. Nam, T. Katayama, and A. Nilsson*, “Maxima in the Thermodynamic Response and Correlation Functions of Deeply Supercooled Water”, Science, 358, 1589-1593 (2017)