Here’s what you’ll learn when you read this story:

• Scientists have long suspected that supercooled water has a hidden critical point, but it’s been nearly impossible to observe experimentally.
• Using an X-ray free-electron laser, researchers have now confirmed that water can exist in two distinct liquid forms that merge into one at extreme cold and pressure.
• This discovery may help explain why water behaves so differently from other liquids.

Water makes life on Earth possible, so you’d think scientists would know all the ins-and-outs of how this vital liquid works.

Right?

Well, think again. Dating back to the work of German physicist Wilhelm Röntgen in the 1890s, it’s been known that your average run-of-the-mill water is one of the weirdest liquids on the planet. According to Chemistry World, H₂0 exhibits 66 properties that differ from other liquids, including its high surface tension, heat capacity, boiling point, and density.

“If you look at the simple thermodynamic and kinetic properties of liquids, as you change pressure and temperature, they all behave the same way. Except for water.” Anders Nilsson, a researcher from the University of Stockholm who has extensively studied the strange properties of water for more than a decade, told Chemistry World in 2020.

To try to understand why water behaves this way, Nilsson has been in search of what’s known as the liquid-liquid critical point, or LLCP. We’ve long known about water’s high critical point of around 374 degrees Celsius and 218 times atmospheric pressure, where the difference between liquid and gas disappears. But scientists hypothesized that there’s a critical point when water is supercooled, meaning that it remains liquid at temperatures well below its normal freezing point. One reason scientists expect this critical point to exist is that water reaches its maximum density at 4 degrees Celsius before reversing course—a behavior that also explains why ice floats. This anomaly in water’s density could be explained by a sub-zero critical point, believed to be somewhere between -40 and -70 degrees Celsius.

In 2020, Nilsson was part of a team that discovered that supercooled water can exist as two distinct phases—a high-density liquid and a low-density liquid—supporting the idea that these phases merge at the much sought-after LLCP. However, identifying that exact point isn’t easy as water freezes faster than most measurement methods can capture.

Most, but not all.

In a new study published in the journal Science, Nilsson and a team led by Kyung Hwan Kim at the Pohang Accelerator Laboratory used extremely fast bursts from an X-ray free-electron laser to observe supercooled water as it transitioned into ice. The scientists revealed that water’s LLCP occurs at around 210 kelvins, or -63 degrees Celsius. In other words, Röntgen’s initial investigations into the strange properties of water some 130 years ago were verified using his other world-changing discovery: X-rays.

“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 said in a press statement. “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.”

Just as with water’s higher critical point, observing water’s LLCP is only possible under immense pressure (roughly 1000 atmospheres), but as conditions change, the two phases eventually merge into one. Interestingly, these fluctuations likely extend across temperatures and pressures, including into normal environmental conditions, which could explain why water is so weird. Now that scientists have pinpointed this critical point, it could have far-reaching impacts in a variety of fields.

“There has been an intense debate about the origin of the strange properties of water for over a century since the early work of Wolfgang Röntgen,” Nilsson said in a press statement. “The next stage is to find the implications of these findings on water’s importance in physical, chemical, biological, geological and climate-related processes. A big challenge in the next few years.”