The Pioneer Vermilion Iron Mining Company opened the Pioneer Mine in 1888 and began shipping ore the following year. When Oliver Iron Mining Company leased the Pioneer in 1898, it was producing over 500,000 tons annually. Eventually, it was considered the richest of Minnesota's underground mines. It closed in 1967 not because the ore body was exhausted, but because underground mining was so labor-intensive. With a 100-man workforce, open pit mines produced about 5,000,000 tons of ore annually, while the underground Pioneer operations employed approximately 600 miners to produce 1,000,000 tons annually. Working conditions were extremely hazardous, with constant danger that water would soak the ground above, break through and rush into the mine, sometimes to a depth of 200 feet, killing all those not fast enough to escape. Cave-ins, mud-slides, and premature dynamite blasts also caused injuries or fatalities. Remaining mine structures reflect the Pioneer Mine's glory days, including the captains' and miners' dries, where employees changed from work clothes and showered; the shaft house, which provided weather protection; the stack, built in 1902 to create a draft for boilers powering early steam hoists; the 1927 engine house powering later electric hoists; the stead headframe; and a water tower.