Ice cream comes in many different flavors. But even pure ice, which consists only of water molecules, has been discovered to exist in more than 20 different solid forms or phases that differ in the arrangement of the molecules. The phases are named with Roman numerals, like ice I, ice II or ice III. Now, researchers led by scientists from the Korea Research Institute of Standards and Science (KRISS) have identified and described a new phase called ice XXI. The results are published in the journal Nature Materials.