Scientists just discovered a new kind of ice 'XXI' and it's nothing like the one in your freezer

Water is one of the simplest molecules known to science, yet it continues to surprise researchers. Under normal conditions, it freezes into the familiar form of ice that floats on lakes and rivers. But when water is exposed to extremely high pressures, its molecules behave in unexpected ways. They rearrange into different crystalline structures, creating entirely new forms of ice .



These forms, called ice phases, show how hydrogen bonds, the tiny links connecting water molecules, can bend, twist, and compress under stress. Understanding these changes helps scientists explain how water behaves deep inside icy planets and moons, where pressure reaches millions of times that of Earth ’s atmosphere.








Why does water behave so strangely when it freezes



A recent study published in Nature Materials revealed that when water is squeezed to enormous pressures, it can freeze and melt through several unexpected pathways. Using a diamond anvil cell, a device that compresses water between two diamonds, along with ultra-fast X-ray imaging, researchers found that water doesn’t form a single type of ice under pressure. Instead, it can briefly pass through multiple temporary phases before stabilising into a known structure called ice VI.



Among these short-lived forms were two new types: ice XXI and a metastable version of ice VII . These “in-between” ices existed only for microseconds, but they revealed that water molecules can organise themselves in more ways than previously thought. This means that even at room temperature, water can crystallise, melt, and recrystallise through several different routes depending on how quickly and how strongly it is compressed.








How do metastable ices form and what makes them special



To picture how these forms appear, imagine water molecules connected by flexible hydrogen bonds that act like springs. At normal pressure, each molecule bonds with four others, forming an open lattice that gives ice its light structure. When compressed, those bonds shorten and shift, forcing molecules into denser patterns.



Sometimes, water doesn’t jump straight to its most stable form. Instead, it pauses at an intermediate state called metastable ice. This temporary structure holds more energy and exists only under specific conditions. The Nature Materials study found that as pressure reached over a billion pascals, around 10,000 times the pressure at the bottom of the ocean, water could solidify in five different ways.



One of these forms, ice XXI , displayed a body-centred tetragonal arrangement, meaning the molecules packed more tightly than in any previous ice type. Though it lasted only moments, it showed that water can take several “routes” toward freezing, depending on how fast pressure builds. This behaviour supports a principle known as Ostwald’s step rule, which says materials often pass through less stable states before settling into equilibrium.







How do scientists capture such fast changes?



Watching water turn into different types of ice in real time requires cutting-edge tools. The researchers used an ultrafast X-ray laser that captured molecular changes at millionth-of-a-second intervals. By combining this with rapid compression cycles in the diamond anvil cell, they recorded how water molecules shifted positions during freezing and melting.



Their images showed that water doesn’t always freeze uniformly. In some regions, one type of ice forms while another appears nearby, and the two can merge or transform into new structures as conditions fluctuate. This dynamic view revealed that water’s internal structure is constantly rearranging itself, even when it appears solid from the outside.



Computer simulations supported the observations. They suggested that high-pressure liquid water transforms into a denser version known as very-high-density water. This stage alters how hydrogen bonds bend and rotate, making it possible for metastable phases like ice XXI to appear. The models also explained why ice VII sometimes competes with ice VI when water is rapidly compressed, showing just how many paths freezing can take.








Why do these discoveries matter beyond the lab



At first glance, this might sound like a purely academic curiosity. But these findings have far-reaching implications. Deep inside icy planets such as Neptune and Uranus, or the frozen moons of Jupiter and Saturn, water exists at pressures similar to those recreated in the lab. Knowing which types of ice can form there helps scientists understand the internal structure and heat movement of these worlds.



For example, certain ice phases conduct electricity differently or trap heat in unique ways, influencing a planet’s magnetic field and potential for subsurface oceans. On Earth, studying metastable materials also helps chemists and physicists understand how substances change state: knowledge that can be applied to materials design, crystallisation processes, and even biological systems like protein folding.



The discovery also demonstrates the remarkable progress of modern experimental physics. For decades, the idea that scientists could “see” atoms rearranging in real time was only theoretical. Now, with advanced X-ray imaging and pressure control, those molecular shifts can be recorded as they happen, revealing nature’s mechanisms in exquisite detail.







What does this tell us about the true nature of water



The findings add a new layer to our understanding of a substance we thought we knew well. Water is far from simple; it can freeze into more than 20 known crystal forms, and this number may continue to grow as new pressures and temperatures are explored.



The identification of ice XXI and the multiple freezing–melting routes shows that even a common liquid can behave in intricate and unpredictable ways when placed under extreme stress. These results refine our picture of how hydrogen bonds rearrange, how energy moves between molecules, and how materials navigate from one form to another.



In everyday life, ice might seem ordinary. But deep within its molecular structure lies a universe of patterns, movements, and transformations that still challenge scientific understanding. The next time water freezes, whether in a freezer or on a distant planet, it may be following one of many hidden paths; each shaped by the invisible dance of its hydrogen bonds.













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