Water behaves in ways that defy simple explanation. Unlike most liquids, it reaches its maximum density not at its freezing point, but at 4 °C — a property that plays a crucial role in stabilising aquatic ecosystems and Earth’s climate. For decades, scientists have suspected that such anomalies arise because water can exist in two distinct liquid states. New research published in Science provides some of the strongest experimental evidence yet for that idea.
The group reports what may be the first direct observation of water’s elusive liquid-liquid critical point (LLCP) — a hypothesised condition under which two forms of liquid water become indistinguishable. A team led by Professor Kyung Hwan Kim at POSTECH, working with Professor Anders Nilsson at Stockholm University, reported the findings.
Probing “no-man’s-land”
The LLCP hypothesis has long been proposed as a unifying explanation for water’s unusual physical properties. However, testing it experimentally has proved extremely difficult.
The critical region is thought to lie in a deeply supercooled regime — between roughly −40 °C and −70 °C — sometimes referred to as “no-man’s-land”. In this temperature range, water crystallises into ice almost instantly, preventing conventional measurements.
To overcome this barrier, the researchers used an X-ray free-electron laser (XFEL), capable of capturing molecular-scale changes on timescales of trillionths of a second. Experiments were carried out at the PAL-XFEL facility in South Korea.
Over a decade, the team progressively extended the accessible temperature range. In earlier studies, they demonstrated measurements down to −45 °C, and later to −70 °C using amorphous ice — work that already hinted at the presence of two distinct liquid states.
Evidence of two liquid phases
In the latest study, the researchers tracked structural changes in water across a range of temperatures and pressures with unprecedented precision. They report observing a critical point near −60 °C, where two distinct liquid phases appear to merge into a single supercritical state.
If confirmed, this would provide a physical basis for understanding many of water’s anomalies — including its density maximum, compressibility, and unusual heat capacity.
However, while the findings represent a major advance, the picture is not yet fully settled. The existence and exact nature of the LLCP remain subjects of ongoing scientific debate, and further independent verification will be needed.
Why it matters beyond fundamental physics
Understanding water’s behaviour at a fundamental level has implications far beyond theoretical chemistry.
Water’s density anomaly — linked to the fact that ice floats — helps prevent lakes and oceans from freezing solid, enabling aquatic life to survive in cold climates. More broadly, the structure and dynamics of water influence everything from protein folding in living organisms to the formation of clouds and ice in Earth’s atmosphere.
Improved models of water could therefore feed into fields as diverse as climate science, cryopreservation, and planetary science, where water exists under extreme conditions.
A long-standing question, still being contested
The idea that water might exist in two liquid forms has been debated for decades. By pushing experimental techniques into previously inaccessible regimes, the new study brings researchers closer to resolving that question.
Rather than closing the debate, however, it marks an important step forward — turning a largely theoretical framework into something that can now be probed experimentally.
As further studies test and refine these findings, water — one of the most familiar substances on Earth — continues to reveal unexpected complexity.
