For example, when soil contaminated with metals is mixed with water and lime ─ a white powder produced from limestone ─ a reaction changes the metals into a form that will not dissolve in water. Additives can be mixed into the waste while still in the ground (often referred to as “in situ”). This usually involves drilling holes using cranes with large mixers or augers, which both inject the additives underground and mix them with the waste. The number of holes needed depends on the size of the augers and the contaminated area. Dozens of holes may need to be drilled. When the waste is shallow enough, the contaminated soil or waste is excavated and additives are mixed with it above ground (often referred to as “ex situ”). The waste is either mixed using backhoes and front end loaders or placed in machines called “pug mills.” Pug mills can grind and mix materials at the same time.