Water ice, the unlikely solid that is less dense than its liquid form, is already a thing that should not be. As temperatures drop below freezing, water molecules form super-resilient hexagonal crystals, leaving a material that is technically classifiable as a mineral. But ice only gets stranger from there.
As described in the current issue Nature, researchers have discovered/developed the least dense form of ice so-far observed: Ice XVI. Indeed, it's hardly the first alternative phase of ice, falling in line between 15 others, a family whose members are distinguished by different densities and different crystalline structures.
Ice XVI is what's known as a clathrate hydrate. Its molecules are arranged such that the material is full of tiny gaps, or "cages," that contain gas molecules; clarthrates are known, for example, to hold vast amounts of methane hostage deep under Earth's oceans. Ice XVI is different in that its spaces are empty, the result of leeching neon gas molecules out of the material using rings of water molecules.
Ice XVI is hardly the end of it, however. While the stuff hasn't been observed yet, it's thought that water under enough pressure will turn into a full-on metal. The problem in creating it artificially is that applying that kind of pressure also involves a bunch of heat, so the ice melts before it can change phase.
Meanwhile, though ice on Earth is a crystal, it's not everywhere. In fact, in most of the universe, water is dominated by amorphous ice. This is a phase characterized by disorder, analogous to the structure within common glass, and found mostly in the interstellar medium's frigid void. It occurs when water is subject to a sudden wave of extreme cold, causing a freeze that happens too fast for crystals to form.