Chilean state copper miner Codelco's El Teniente division is the business unit responsible for the underground copper deposit El Teniente, located in Chile's O'Higgins region (VI), 50km from Rancagua city. It is the world's largest underground copper mine, with 3,000km of underground tunnels and a production of over 22Mt of fine copper since startup. The operation comprises the underground production units Diablo Regimiento Fase IV, Esmeralda, Dacita, Reservas Norte, Pipa Norte, Sur Andes Pipa, Pilar Norte and Teniente 4 Sur, and the open-pit deposit Rajo Sur, as well as the Colón concentrator and the Caletones smelter. The smelter produces copper anodes, molybdenum and sulphuric acid. The division is currently developing two projects to add new reserves: Pacífico Sur and Recursos Norte. Codelco will invest US$223mn in Pacífico Sur, a deposit with estimated reserves of 23Mt, a grade of 0.87%, and a potential production of 15,000t of copper ore per day. Recursos Norte, will require a US$485mn investment, with reserves of 109Mt and a potential production of 25,000t/d. El Teniente mine began operations in 1905, with the creation of Braden Copper Company. In 1976, the company, known as Sociedad Minera El Teniente, became one of Codelco's divisions, ten years after the Chilean government began the copper nationalization process. El Teniente is headquartered in Rancagua.