WATER has long been a confounding substance to chemists. This most basic stuff of life is denser when liquid than they might have expected and lighter when solid. It behaves sometimes like a lubricant, sometimes like a glue. It expands both when heated and when cooled. And when cooled beyond the liquid-to-solid transition, it can form a number of different forms of ice. This week, that number rose from 16 to 17.

This 17th form, called ice XVI (the confounding factors of water stretch also into ice nomenclature; ice I, the familiar kind, has two variants), is what is known as a clathrate. Discovered either in the late 18th century by Joseph Priestley or in the early 19th by Humphry Davy (for history too is confoundingly water-marked), a clathrate is an arrangement of molecules that surrounds or encapsulates some other molecule. A clathrate hydrate, then, is just such a cage, but made of water.

Such clathrates were of little more than academic interest until recent years, when it became clear that clusters of them, each wrapped around hydrocarbons such as methane, were gumming up oil and gas pipelines. It is also known that methane clathrates are rife in seafloor sediments around the world. That gets the oil-and-gas types thinking about how to extract the methane molecules from their watery cages, replacing them with the carbon dioxide formed when methane burns—a kind of zero-carbon-sum energy scheme.

The problem is that very little is known about the fine structure or the chemical proclivities of such clathrates, or indeed many of the phases of ice that form under specific combinations of pressure and temperature. Cracking the problem of, for example, methane-for-CO2 exchange requires knowledge of all the intermediate chemical stages. "The hydrate everyone was considering theoretically would be the empty one—just the cage without the gas," says Thomas Hansen, a researcher at Institut Laue-Langevin (ILL), in France, and one of the authors of a paper in Nature this week that outlines ice XVI.

But making the cage without the gas turns out to be a tricky proposition. Clathrates are not all that stable, even when shored up from within by a gas molecule. An atom-thick skin of water with nothing inside seemed improbable.

Together with researchers from the University of Goettingen, in Germany, Dr Hansen used a simple technique to see if the comparatively small atom neon could be teased out of a clathrate by using a simple tool: a vacuum pump. Over the course of five days of continuous pumping, it was hoped the neon would be completely removed from the clathrates. To check that, the team reverted to a much more complex piece of hardware: the ILL's world-beating neutron source, seen in the image above. Because neutrons bounce off hydrogen atoms (representing two out of three atoms in the leftover watery cage) better than the more frequently used X-rays, they were an apposite choice to test whether, in the absence of neon, the clathrate's structure had been maintained.

It had. In removing the neon, the team had created a valuable test-bed for studies with other gases, and in so doing they had created a never-before-seen form of ice. And odd stuff it is too: it is just four-fifths the density of the kind found in your freezer. Also, it is ever-so-slightly larger once the neon atom is removed; as Dr Hansen puts it, "it is as if nothing takes up space." Water still has a few mysteries for the chemists, and perhaps a few tricks for the environmentalists too.