An iron–hydrogen alloy, also known as an iron hydride, is an alloy of iron and hydrogen formed when hydrogen atoms dissolve in the crystal structure of metallic iron. This article deals with two broad classes of the iron-hydrogen system: solid iron at ordinary pressure that can incorporate a small amount of hydrogen into its crystal structure; and iron-hydrogen phases that exist at extreme temperatures and pressures, such as might be found in the Earth's core. These substances are the subject of study in industrial metallurgy and planetary geology.