SEOUL – A joint research team has finally solved a decades-old mystery about water, confirming it can exist in two distinct liquid states at extremely cold temperatures. Researchers from Pohang University of Science and Technology (POSTECH) in South Korea and Stockholm University in Sweden used powerful X-ray lasers to pinpoint water's "liquid-liquid critical point" at approximately minus 60 degrees Celsius and 1,000 times atmospheric pressure. The findings, published in the journal Science on March 26, 2026, offer a fundamental explanation for many of water's unusual properties.
Water's Strange Behavior Unraveled
Water is essential for life, yet it behaves unlike almost any other liquid. Most liquids become denser as they cool, but water reaches its maximum density at 4 degrees Celsius. Below this temperature, it expands, which is why ice floats and lakes freeze from the top down. This strange defiance has puzzled scientists for over a century. For about 30 years, a leading theory suggested these anomalies stemmed from water's ability to switch between two different liquid forms: a high-density liquid where molecules are packed tightly, and a low-density liquid with looser arrangements. The existence of a "critical point" where these two liquid phases become indistinguishable was predicted but never directly observed until now.[princeton+10]
Professor Anders Nilsson of Stockholm University, a lead researcher on the team, said, "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."
Piercing "No Man's Land"
Studying water in its supercooled state, below its normal freezing point of 0 degrees Celsius, presents immense challenges. Between minus 40 and minus 70 degrees Celsius, water quickly crystallizes into ice, a region scientists call "No Man's Land" because its liquid properties vanish almost instantly. This rapid freezing made experimental observation extremely difficult. Previous research often relied on computer simulations to explore this elusive state.[m+2]
To overcome this barrier, the joint team employed advanced technology at the Pohang Accelerator Laboratory in South Korea. They used a fourth-generation X-ray Free Electron Laser, a facility that generates light billions of times brighter than the sun. This allowed researchers to capture molecular movement in a millionth of a second.[m+2]
The Experimental Breakthrough
The experimental method involved spraying microscopic droplets of water into a vacuum. The researchers then used a laser to melt ice into a liquid for a fleeting moment, creating supercooled water. By probing these ultra-short-lived liquid states with X-ray pulses, they could observe the molecular structure and density before it froze. This precise technique allowed them to map out how water fluctuates between its two liquid states.[m+3]
Professor Kim Kyung-hwan at POSTECH, who led the Korean team, has pursued this critical point for the past decade. His team first measured unfrozen water below minus 45 degrees Celsius in 2017 and extended observations to minus 70 degrees Celsius by 2020, showing that water could indeed have two liquid states. In this latest study, they directly captured the point where these two liquid states merge into one, confirming the liquid-liquid critical point near minus 60 degrees Celsius.[biz+2]
"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," Nilsson explained.
Broad Implications for Science
This discovery is a fundamental shift in molecular physics and offers a definitive answer to a historical scientific mystery. The presence of this critical point at low temperatures explains why water's properties, such as compressibility and heat capacity, become increasingly strange as it cools. The fluctuations between the two liquid states are what give water its unique characteristics.[m+3]
The breakthrough opens new doors for research in various fields. It could lead to a better understanding of climate patterns, particularly how clouds form and produce ice. It also has implications for the preservation of biological tissues, as cells can expand and break when frozen, and for understanding the fundamental stability of proteins. Some researchers even suggest it could impact studies related to the origin of life itself.[m+5]
The team plans further experiments in May to refine their current data, which carries a margin of error of 8 degrees. This ongoing work aims for even greater precision in mapping the most essential substance in the universe.[m]
