Helium is the only element that remains liquid at absolute zero under normal atmospheric pressure.

Helium's resistance to freezing stems from zero-point energy, a quantum mechanical phenomenon. According to the laws of quantum mechanics, atoms never cease moving completely, even at absolute zero. Because helium atoms are extraordinarily light and the intermolecular forces between them are exceptionally weak, this residual quantum motion prevents them from locking into a solid lattice structure.This remarkable property was first documented by Dutch physicist Heike Kamerlingh Onnes, who liquefied helium in 1908. Despite cooling the substance to temperatures as low as 1.5 Kelvin, he observed that it remained in the liquid state. It was not until 1926 that his student, Willem Hendrik Keesom, successfully solidified helium by applying substantial external pressure—approximately 25 atmospheres—forcing the atoms into a solid configuration.Helium exhibits two distinct liquid phases: Helium I and Helium II. Below 2.17 Kelvin, helium becomes a superfluid with zero viscosity, enabling it to flow without friction and even climb the walls of containers. This extraordinary behavior results from Bose-Einstein condensation occurring at a macroscopic scale. No other element on the periodic table demonstrates this unique combination of liquid stability and superfluidity at such extreme temperatures.