Researchers at Stockholm University have been able to follow the transformation between two distinct different liquid states of water, both being made of H2O molecules. Using X-ray lasers, at around -63 Centigrade the two liquids exist at different pressure regimes with a density difference of 20%.
By rapidly varying the pressure before the sample could freeze, it was possible to observe one liquid changing into the other in real-time. Water is often called "anomalous." If water would have behaved like a "normal liquid" we would not exist.
There have been a number of explanations to the weird properties of water. One of them proposes that water has the ability to exist as two different liquids at different pressures and at low temperatures. If we would be able to keep the two liquids in a glass they would separate with a clear interface in between, similar to water and oil.
Ordinary water under ambient conditions is only one liquid and no interface would be seen in a glass. On a molecular level though, it fluctuates, creating small local regions of similar density as the two liquids, causing water's strange behaviour.
The challenge has been that no experiment has been possible at the temperatures where the two liquids would co-exist since the ice would form almost instantaneously. Until now, it has only been possible to investigate water at these conditions using different types of computer simulations, which has led to a lot of contradicting results depending on the model used.
Source: Stockholm University