Rare earth elements (REEs) are a group of 17 chemically similar metals, which got their name because they typically occur at low concentrations (between 0.5 and 67 parts per million) within Earth's crust. Because they are indispensable in modern technology such as light-emitting diodes, mobile phones, electromotors, wind turbines, hard disks, cameras, magnets and low-energy lightbulbs, the demand for them has increased steadily over the past few decades, and is predicted to rise further by 2030.