Helium Is the Only Element That Cannot Be Solidified by Pressure Alone

While most substances can be forced into a solid state by simply squeezing their atoms together, helium defies this rule due to a quantum mechanical effect called zero-point energy. Even at the theoretical temperature of absolute zero, where all classical atomic motion should cease, helium atoms continue to vibrate with so much energy that they resist locking into a fixed, solid structure. The weak attractive forces between these incredibly lightweight atoms are simply not strong enough to overcome this perpetual quantum jiggle, no matter how much external pressure is applied.

To finally coax helium into a solid, scientists must tackle the problem from two fronts. Not only do they need to apply significant pressure—about 25 times that of our atmosphere—but they must also chill the element to temperatures below -272°C, just a single degree above absolute zero. This incredible feat was first achieved by Dutch physicist Willem Keesom in 1926, nearly two decades after helium was first liquefied, highlighting the extreme difficulty of pinning down this elusive element.

Even then, solid helium remains profoundly strange. It is one of the few known examples of a "supersolid," a bizarre state of matter that possesses a rigid, crystalline lattice like any other solid, yet can also flow without any friction, a property of superfluids. This means parts of the solid can move through other parts as if they weren't even there, a direct and visible manifestation of the quantum laws that govern helium's unique behavior.