Despite our utter dependence on it, water is a mystery. Scientists don't understand why it expands as it cools, why hot water turns to ice faster than cold water or its extraordinary surface tension, which allows it to cling to soil and support insects. "It's almost like water is trying to hide its secrets," says John Russo, a mathematician at the University of Bristol in England, who thinks he may have figured one out.
Most scientists seeking to parse water's oddities expose it to extreme conditions, such as very low temperatures. Russo and colleagues at the University of Tokyo took a different approach, creating computer models of "extreme water," as he describes it. In their simulation, they altered the bonds that hold the molecules together until they began behaving like a more normal liquid. "We switched off the strangeness," says Russo. The team could then see what bond structures were necessary for the anomalous behavior. When simulated ice no longer floated, for example, the team knew they'd eliminated the structure responsible for that peculiar trait.
Their conclusion was as surprising as their subject. "You should really think of water as two separate liquids mixed together," says Russo. The difference between them: the hydrogen bonds holding the molecules together. In one, four are arranged in an organized, symmetrical tetrahedron—a pyramid with four faces. In the other, the pyramids are disordered and interconnected. Water, it seems, is equal parts order and chaos—a poetic result for Earth's most valuable resource.
The finding, published in April in the Proceedings of the National Academy of Sciences, could influence the climate change models that predict how warming temperatures will alter our world. "Water matters for the basic physics of the way climate works," says Chris Milly, a hydrologist with the National Oceanic and Atmospheric Administration. The way water crystals form in clouds is not well understood, for example, yet the crystals determine how much sunlight clouds will reflect back into space, a calculation that matters when it comes to modeling future warming.
Milly says explaining water's weird behavior isn't as important as simply knowing that it exists, which means the two-liquid theory may not improve climate models. But that's just one potential application. Cryopreservation (freezing tissue) and many fundamental biological processes rely on water's bizarre properties. Russo hopes further research will reveal more secrets.
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