Water gets stranger the colder it gets, and researchers now say they have pinned down a hidden turning point in supercooled water that refuses to freeze.

In a new study, an international team reports more direct evidence that water can split into two liquid phases under extreme conditions, one high-density liquid and one low-density liquid. The researchers also found evidence of a critical point beyond which those two phases give way to a single liquid state with highly unstable molecular structure.

Scientists have long theorised about this behaviour in supercooled water, liquid water kept well below its usual freezing point through pressure and temperature controls. But getting a clear look at it has been difficult because these states sit right at the edge of freezing. The study describes that region as a kind of “no man’s land” for measurements.

“What was special was that we were able to X-ray unimaginably fast before the ice froze and could observe how the liquid-liquid transition vanishes and a new critical state emerges,” said chemical physicist Anders Nilsson, from Stockholm University in Sweden.

“For decades there has been speculations and different theories to explain these remarkable properties and one theory has been the existence of a critical point. Now we have found that such a point exists.”

To capture the changes, the team used rapid heating with infrared lasers and ultra-fast X-ray snapshots. The researchers engineered ice, then pushed it through the liquid-liquid state, across the critical point, and into the fluctuating single-liquid state while tracking what happened on extremely short timescales.

The exact location of the critical point still has not been pinned down, but the new work narrows the search. The researchers think it sits around minus 63 degrees Celsius and about 1,000 atmospheres of pressure.

The study also found that the critical point behaves in a way the researchers compared to a black hole. As water gets closer to that point, the liquid’s internal dynamics slow down and changes in structure take much longer. According to the researchers, that means the liquid cannot avoid the transition.

The findings add to a long list of odd behaviour in water. One familiar example shows up in a glass. Most matter shrinks and becomes denser as it cools, but water does not behave that way, which is why ice cubes float instead of sinking.

The researchers say this latest work gives physicists a stronger basis for explaining those unusual properties. It also points to wider questions, because water is involved in physical, chemical, biological, geological, and climate-related processes.

“Researchers studying the physics of water can now settle on the model that water has a critical point in the supercooled regime,” Nilsson said.

“The next stage is to find the implications of these findings on water’s importance in physical, chemical, biological, geological, and climate-related processes.”

The study also points to another reason water keeps attracting attention. The researchers said water differs from other liquids not only because of its unusual physics, but also because it is essential to life, as far as we know.

“I find it very exciting that water is the only supercritical liquid at ambient conditions where life exists and we also know there is no life without water,” said chemical physicist Fivos Perakis, from Stockholm University.

The research has been published in Science.