Ice that doesn’t melt when it’s warm: Meet Ice XXI

Scientists have discovered a never-before-seen type of ice — 'Ice XXI' — that stays solid even at room temperature under extreme pressure.

Scientists have developed an ice using pressure and X-rays.

Scientists have developed an ice using pressure, temperature, and X-ray experiments (AI image used for representation only).

Picture this: ice that doesn’t melt even when it’s warm. That’s not science fiction anymore — it’s now a science fact. Researchers have just discovered a completely new type of ice that stays solid at room temperature when squeezed under enormous pressure. And it could change the way we understand water forever.

Water is way more complicated than it looks

We all know the basics — water can be liquid, vapour, or ice. But scientists understand that ice itself comes in more than 20 different forms, depending on temperature and pressure. Each version has its own unique structure, like different patterns made by the same Lego blocks.

This new study, backed by Korea Research Institute of Standards and Science and the National Research Council of Science and Technology, has found a hidden version of ice that forms under intense pressure, even when it’s warm enough for normal ice to melt instantly.

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What was the experiment?

To make this discovery, researchers used a machine called a diamond anvil cell—imagine two diamonds pressing water with incredible force. They squeezed and released the water hundreds of times at various speeds while monitoring the entire process with ultra-fast X-rays [X-ray free-electron laser (XFEL)].

Each time, they watched how the water froze, melted, and reshaped itself. The result? Not one, but five different ways for water to turn into ice and back again.

How ‘Ice XXI’ came to being

During one of those pressurising sessions, scientists spotted something extraordinary — a never-before-seen kind of ice. They called it ‘Ice XXI’. Unlike any known form, Ice XXI has a massive, perfectly ordered structure that keeps it stable even at room temperature, as long as it’s under roughly 1.6 gigapascals of pressure. This is equivalent to nearly 16,000 times the Earth’s atmospheric pressure.

In simple terms, it’s a type of ice that shouldn’t exist, yet it does.

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Five surprising ways water freezes

The researchers found that water has at least five distinct freezing and melting patterns at room temperature:

Directly into Ice VI, then back to water.
Into a temporary state called ice VII before melting.
Ice VII changing to ice VI before melting.
Through the new Ice XXI before returning to normal.
Through Ice XXI and ice VII before finally reaching ice VI, then melting.

Each pathway depends on how quickly and strongly the water is pressed. Some crystal forms appear in microseconds before transforming again.

Why this matters beyond the lab

This discovery isn’t just cool (pun intended). It could help explain what happens deep inside icy moons, such as Jupiter’s Europa and Ganymede satellites, where water exists under extreme conditions. It also gives scientists valuable data for modeling how water behaves under pressure.