Hydrogen can be stored in its pure form. To achieve a high energy density, hydrogen is stored either in gaseous form and under high pressure or it is liquefied (LH₂). In order for hydrogen to be liquefied, it must be cooled to -253 °C. This means that about one third of the energy stored in hydrogen is used for cooling. On the other hand, LH₂ can be used without further pre-treatment, for example in fuel cells. LH₂ begins to evaporate immediately after cooling is completed. For the storage of gaseous hydrogen, salt caverns are being considered in addition to storage in pressure tanks.