Comstock Lode mining began in 1859 with open pits at Gold Hill to the south and Virginia City to the north. Within months, the Ophir Mine pit at Virginia City's outcropping became dangerously deep, forcing miners to excavate underground.
The Comstock challenged traditional methods of supporting mines with timbers assembled in an inverted "U." The decomposed quartz embracing the ore tended to fold itself around wooden supports, collapsing on excavations. In addition, the ore vein expanded up to sixty feet wide. A single timber supporting such a span would snap.