The Minesota fissure vein was discovered in 1847 when prospectors found a six-ton (5.4 mt) mass of native copper in a pit dug by aboriginal miners. In the pit was growing a hemlock nearly 400 years old by the number of growth rings. Mining began in 1848, and from 1855 through 1862, the Minesota was the most productive copper mine in the United States. The mine had ten shafts, the deepest of which extended to a depth of 1200 feet (366 m). In 1856, miners tunneled into a 527-ton (478 mt) mass of native copper, the second-largest such mass found in the Copper Country. Besides masses of copper recovered through hand-sorting, the mine ran a small stamp mill to recover finer-grained copper in ore that ran 3%.