Water is one of the relatively few compounds in which the solid is less dense than the liquid. That, of course, is why ice floats in your glass. But not all ice is created equal. Researchers today unveiled a new solid phase of ice that’s the lowest density version known. Known as ice XVI, the 17th solid phase of ice discovered to date, it has a cagelike structure that can trap other molecules (green and gray above). Such ice cages, known as clathrates, are known to store enormous quantities of methane on the deep ocean floor. The new clathrate, by contrast, is empty, though it didn’t start that way. The cagelike structure originally formed surrounding neon atoms (blue). The neon was then leached out of the clathrate through rings of water molecules (red dashed lines). The new form of ice may help researchers better understand clathrates in general, and perhaps ease the flow of oil and gas through pipelines at low temperatures.