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, 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 division is currently developing two projects to add new reserves: Pacífico Sur and Recursos Norte. 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.