Abstract
In the perplexing domain of supercooled water, scientists have dedicated decades to unraveling its mysteries. Using advanced sampling methods within the TIP4P/2005 water model, this study pinpoints the second critical point at temperature Tc= 238±2 K and pressure Pc= 288±30 bar, signifying an abrupt first-order to a gradual continuous phase transition. It also reveals a transition temperature Tt=172±1 K at which a pivotal transformation unfolds, marking the stability switching of a two-step nucleation process, unveiling a previously unnoticed mid-density state bridging high- and low-density liquid states. These findings redefine our understanding of liquid-liquid phase transition, contributing to a comprehensive phase diagram for supercooled water including the elusive "no man’s land, unravelling its intricate complexity.