In a recent breakthrough, researchers at KRISS and LLNL uncovered ice XXI, a crystal with 152 molecules per unit cell that eluded earlier computer models. Using diamond‑anvil compression and X‑ray free‑electron lasers, the team captured 15 diffraction spots, revealing a structure far larger than the 75,000‑phase catalog.

The discovery challenges the assumption that simulations can predict all viable ice forms. Ice XXI’s appearance in a metastable sequence—water to ice XXI to ice VII—illustrates Ostwald’s step rule in action, showing that systems can favor easier‑to‑form, less stable intermediates.

This finding underscores water’s molecular flexibility and hints at exotic ices that could exist in planetary cores or comet tails. By refining experimental techniques, scientists now have a clearer map of potential ice phases, informing models of planetary interiors and high‑pressure chemistry.

Ultimately, ice XXI proves that even well‑studied substances can harbor unexpected complexity when probed under extreme conditions.