Physically removing the rock ore generally involves either open-pit mining or underground mining. Milling is the process that removes uranium from the ore, which is mostly obtained in open-pit and underground mines. Once at the mill, the ore is crushed and ground up, and treated with chemical solutions to dissolve the uranium, which is then recovered from the solution. Tailings are the wastes from the millings processes and are stored in mill tailings impoundments, a specially designed waste disposal facility. Since 1979, when uranium mine workers began being diagnosed with lung diseases, such as cancer, regulators have gradually tightened controls and mandated improved uranium mining practices. Recently, officials also have become concerned with the broader impacts of uranium mining on public health and the environment. Workers are directly exposed to the radiation hazards of uranium mines. Uranium mining also releases radon from the ground into the atmosphere.