However, once they discovered that better quality lead could be found by sloping downward, the company decided to start working underground. The company also acquired a diamond drill in 1869 which sped up production. Improvements smelting furnaces increased production even further. As more ore was extracted, it was apparent to the company that getting it to market faster was imperative (up to that point ore was transported by wagons). As a result, the company built a railroad line to the train station in the town of Summit to connect with the St. Louis, Iron Mountain and Southern Railway.
In the coming years, the company bought more land and built additional furnaces. In 1887, the company built its own rail line to the town of Herculaneum, which is located south of St. Louis along the Mississippi River. A large smelter was built there as well. During the 1890s, the company merged with other mining companies. Despite increased competition, the company thrived in the coming decades, growing bigger by buying more property and modernizing production. It even started mines in Peru and Argentina. The company played an important role in providing lead for the American military during World War I and II. By 1962, however, the Bonne Terre Mine was depleted and was closed. During the peak of its operation between 1870 and 1943, the mine produced $300 million dollars worth of lead. The company continued to operate in the 1970s before it was acquired by another company in the early 1980s.