Hydrogen has been the prototypical system of the insulator-to-metal transition ever since it was predicted in 1935 that the insulating molecular solid would transform to a conducting monatomic solid at sufficiently high pressure and density at absolute zero temperature (0 K). So far, metallization of solid hydrogen has not been observed. However, fluid hydrogen has recently been observed to become metallic at pressures above 140 gigapascals (1.4 megabars) and temperatures of about 3000 K (4900°F).