Unlike any other element, helium will remain liquid down to [[absolute zero]] at normal pressures. This is a direct effect of quantum mechanics: specifically, the [[zero point energy]] of the system is too high to allow freezing. Solid helium requires a temperature of 1–1.5 K (about –272 °C or –457 °F) and about 25 bar (2.5 MPa) of pressure.<ref>{{cite web |date = 2005-10-05 |url = http://www.phys.ualberta.ca/~therman/lowtemp/projects1.htm |title = Solid Helium |publisher = Department of Physics [[University of Alberta]]|accessdate=2008-07-20}}</ref> and may be a [[superglass]].<ref>[http://www.news.cornell.edu/stories/April09/superglass.ws.html Supersolid or superglass? Cornell researchers study a strange state of matter in helium]</ref> It is often hard to distinguish solid from liquid helium since the [[refractive index]] of the two phases are nearly the same. The solid has a sharp [[melting point]] and has a [[crystal]]line structure, but it is highly [[Compressibility|compressible]]; applying pressure in a laboratory can decrease its volume by more than 30%.<ref name="LANL.gov">{{RubberBible86th}}</ref> With a [[bulk modulus]] on the order of 5×10<sup>7</sup> [[Pascal (unit)|Pa]]<ref>{{cite journal |author = Malinowska-Adamska, C.; Soma, P.; Tomaszewski, J. |title = Dynamic and thermodynamic properties of solid helium in the reduced all-neighbours approximation of the self-consistent phonon theory |journal = Physica status solidi (b) |volume = 240 |issue = 1 |pages = 55–67 |doi = 10.1002/pssb.200301871 |year = 2003}}</ref> it is 50 times more compressible than water. Solid helium has a density of 0.214 ± 0.006 g/ml at 1.15 K and 66 atm; the projected density at 0 K and 25 bar is 0.187 ± 0.009 g/ml.<ref>{{cite journal |author = Henshaw, D. B. |title = Structure of Solid Helium by Neutron Diffraction |journal = Physical Review Letters |volume = 109 |issue = 2 |pages = 328–330 |doi = 10.1103/PhysRev.109.328 |year = 1958}}</ref>
