More than 20 different crystal structures of ice have been discovered across various temperature and pressure ranges. This is considered an unusually large number of polymorphs for a pure substance. Of these, 12 were discovered by the 20th century, and the remaining 8 have been found since the 21st century, with the pace of discovery remarkably accelerating.

Following the synthesis of plastic ice reported at the beginning of this year, there has been a report of the successful synthesis of ice T2, which our group predicted in 2018.

Ice T2 is a new form of ice that forms when high-pressure water is cooled in computer simulations, with an extremely complex crystal structure. We discovered this structure by chance during the simulation process, but since the temperature and pressure conditions were in the range where ice VI or ice VII would normally form, it was questionable whether such ice could actually be created.

Now, the research team led by Kobayashi at the University of Tokyo has used a new technique of supercooling water (cooling liquid below its freezing point) under ultra-high pressure to discover ice T2 and two additional new forms of ice. If the nomenclature follows the usual pattern, these ices are expected to be called ice XXI (21), XXII (22), and XXIII (23).

Which will find the next ice first - simulation or experiment? How many ice crystal structures will exist by the end of this century? Perhaps some of the other ice forms we predicted might also be synthesized.

• Kobayashi et al. preprint (ArXiv): Hiroki Kobayashi, Kazuki Komatsu, Kenji Mochizuki, Hayate Ito, Koichi Momma, Shinichi Machida, Takanori Hattori, Kunio Hirata, Yoshiaki Kawano, Saori Maki-Yonekura, Kiyofumi Takaba, Koji Yonekura, Qianli Xue, Misaki Sato, Hiroyuki Kagi; New metastable ice phases via supercooled water; https://arxiv.org/abs/2507.14415

• Yagasaki, T., Matsumoto, M. & Tanaka, H. Phase Diagrams of TIP4P/2005, SPC/E, and TIP5P Water at High Pressure. J. Phys. Chem. B 122, 7718–7725 (2018). DOI:10.1021/acs.jpcb.8b04441