The Central Pacific made great progress along the Sacramento Valley. However construction was slowed, first by the foothills of the Sierra Nevada, then by the mountains themselves and most importantly by winter snowstorms. Consequently, the Central Pacific expanded its efforts to hire immigrant laborers (many of whom were Chinese). The immigrants seemed to be more willing to tolerate the horrible conditions, and progress continued. The increasing necessity for tunnelling then began to slow progress of the line yet again. To combat this, Central Pacific began to use the newly invented and very unstable nitro-glycerine explosives—which accelerated both the rate of construction and the mortality of the Chinese laborers. Appalled by the losses, the Central Pacific began to use less volatile explosives, and developed a method of placing the explosives in which the Chinese blasters worked from large suspended baskets that were rapidly pulled to safety after the fuses were lit.