Ice XXI reveals five pathways to solid water at room temperature

An international team compressed liquid water at room temperature inside a dynamic diamond‑anvil cell, uncovering five freezing/melting pathways that can bypass the simple ice VI route and revealing a new metastable crystal, ice XXI, containing 152 water molecules per repeating unit. The work refines our understanding of water under extreme pressure and could improve models for high‑pressure environments in icy worlds and water‑rich materials.