Surface mining techniques are used when a vein of coal or other substance lies so close to the surface that it can be mined with bulldozers, power shovels, and trucks instead of using deep shaft mines, explosive devices, or coal gasification techniques. Surface mining is especially useful when the rock contains so little of the ore being mined that conventional techniques, such as tunneling along veins, cannot be used. Surface mining removes the earth and rock that lies above the coal or mineral seam and places the overburden off to one side as spoil. The exposed ore is removed and preliminary processing is done on-site or the ore is taken by truck to processing plants. After the mining operations are complete, the surface can be recontoured, restored, and reclaimed. Surface mining already accounts for over 60% of the world's total mineral production, and the percentage is increasing substantially. Many factors contribute to the popularity of surface mining. The lead time for developing a surface mine averages four years, as opposed to eight years for underground mines. The productivity of workers at surface mines is three times greater than that of workers in underground mining operations.