Located in Chile, El Teniente is the world's largest underground mine for what metal?

El Teniente ("The Lieutenant") is an underground copper mine in the Chilean commune of Machalí in Cachapoal Province, Libertador General Bernardo O'Higgins Region, near the town of Sewell, 2,300 m (7,500 ft) above mean sea level in the Andes.

Mining at El Teniente is reported to have started as early as 1819. Kennecott Copper Corporation ran the mine through a subsidiary company up until the Chilean nationalization of copper and the formation of the state owned copper mining company Codelco, who currently operates the mine. With over 3,000 kilometres (1,900 mi) of underground drifts, it is reported as "the world's biggest underground copper mine", and is the largest of Codelco's operations.

El Teniente mine is undergoing, since 2011, a structural project called New Mine Level (NML). It consists of expanding the mine deeper into the hill at 1,880 meters above sea level. The project is being carried out without interrupting the operation of El Teniente Division.