Nuclear reactors use a certain type of uranium called U-235 as fuel because its atoms are easily split apart. Uranium is the most widely used fuel by nuclear power plants for nuclear fission. Uranium occurs naturally in low concentrations in soil, rock, and water, and is commercially extracted from uranium-bearing minerals such as uraninite. Uranium ore can be mined from open pits or underground excavations. The ore can then be crushed and treated at a mill to separate the valuable uranium from the ore. Uranium may also be dissolved directly from the ore deposits in the ground (in-situ leaching) and pumped to the surface. The uranium concentrate is separated from uranium ore at uranium mills or from a slurry at in-situ leaching facilities. The vast majority of nuclear power reactors use the isotope uranium-235 as fuel, which only makes up 0.7% of the natural uranium mined and must therefore be increased through a process called enrichment. This increases the uranium-235 concentration from 0.7% to between 3% and 5%, which is the level used in most reactors.