Water has a long list of 'anomalous' physical (and chemical) properties (list here, archived from London Southbank University). Most are now fairly well understood - but there is currently no overall model to explain all of its unusual properties.
One anomaly is the existence and prevalence of so called 'water clusters' in volumes of liquid water - it's known that liquid water is not homogeneous at the nanoscopic level. For example, small clusters of four water molecules may come together to form water bicyclo-octamers. Greater numbers can form large clusters; for instance the icosahedral (H2O)280. It should be noted that such clusters are constantly and dynamically forming and re-forming in liquid water. In a sense, the chemical formula H2O is a dramatic over-simplification.
Theoretical models using computer simulations have suggested a myriad of different cluster formations, and some have been observed and confirmed in nature using laser techniques. But little is understood about how water clusters form, persist, and affect the physical and chemical properties of bulk water.
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