According to the UC Davis ChemWiki Chemistry of Helium, helium has a comparatively unusual property, specifically:
Helium is the only element that cannot be solidified by lowering the temperature at ordinary pressures.
'Ordinary' referring to standard air pressure (1 atmosphere). In order to solidify, there needs to be a corresponding pressure increase, with a projected density of:
0.187±0.009 g mL−1 at 0 K and 25 bar.
So why can't helium be solidified at 'ordinary' pressures?
