Helium Refuses to Freeze, Even at Absolute Zero

Drop the temperature low enough and almost everything freezes solid. Helium is the lone holdout. As Los Alamos National Laboratory notes, it remains liquid down to absolute zero (−273.15 °C) at ordinary pressures and will only solidify when you squeeze it.

The culprit is quantum zero-point motion. Even at the coldest temperature physics allows, helium atoms keep jittering with irreducible energy they can never shed. Helium atoms are exceptionally light and barely attract one another, so this restless quantum jiggling is strong enough to keep them from ever locking into a crystal lattice.

To freeze helium, you cannot cool it. You have to crush it.

The only way to make solid helium is to apply roughly 25 atmospheres of pressure, about 25 times the air pressure at sea level, forcing the atoms close enough that their weak bonds finally win out. That makes helium the only known element that cannot be solidified by cooling alone, a vivid demonstration that quantum mechanics governs the everyday world of matter.