Researchers have discovered a completely new type of ice that can form at room temperature when water is subjected to immense pressure.

The discovery, which reveals previously unknown behaviour of one of Earth’s most studied substances, could have major implications for understanding the interiors of distant planets and moons.

The findings were reported in the journal Nature Materials.

The team, working at the European XFEL facility in Germany, used diamond anvil cells to squeeze ordinary water to pressures of up to 2 gigapascals, or about 20,000 times atmospheric pressure. Under these conditions, water molecules rearranged into an entirely new crystalline structure never seen before. The researchers named it Ice XXI.

Unlike regular ice, which forms in hexagonal patterns, Ice XXI has a tetragonal lattice that makes it denser and more stable under extreme pressure. What makes it even more unusual is that it forms without cooling the water, as the process depends solely on compression rather than freezing.

It is the twenty-first type of ice ever recognised. Scientists have already catalogued an incredible variety of forms, including hexagonal, cubic, amorphous, and superionic. Each form emerges from a unique combination of temperature and pressure.

Ice XXI has joined that growing family, demonstrating that water's solid state is significantly more varied than anything found in a freezer.

Scientists also found that water can crystallise in several different ways depending on how quickly the pressure changes. The study observed five distinct freezing pathways, challenging the long-held assumption that water follows a single, predictable path to becoming solid.

This discovery could explain how water behaves deep inside icy moons like Ganymede and Titan, or even on water-rich exoplanets where similar high-pressure environments exist.