Part of state-run Codelco, 55 kilometers east of Rancagua , El Teniente is the world’s largest underground mine; its more than 2,400 kilometers of tunnels yield some 435,000 metric tons of copper ore per annum. It dates from 1905, when the U.S.-based Braden Copper Company began operations; after its acquisition by Kennecott Copper Company and the outbreak of World War I, production increased rapidly and the company became enormously wealthy.
Until the late 1960s, El Teniente’s residential community of Sewell (named for Braden’s first president) was a classic 20th-century company town and a thriving (but residentially segregated) community of North American managers and technicians, their Chilean support staff, a mass of miners and their families, and many single miners as well.
At that time, though, President Eduardo Frei Montalva’s Operación Valle began to transfer all of Sewell’s 15,000 workers and their families to Rancagua  because the prevailing winds carried toxic sulfur hydroxide fumes from the smelter at Caletones.
Parts have been dismantled or demolished, such as the so-called Barrio Americano where North American technicians lived with their families, but many structures, such as the hospital, the Escuela Industrial (Industrial School, now home to the museum), the theater, a four-lane bowling alley (Chile’s first, dating from 1917), and the Club Social, remain intact.
All these were connected not by streets but by a warren of concrete steps and walkways that gave the town its nickname, “Ciudad de las Escaleras” (City of Staircases).
Codelco has nominated Sewell for UNESCO World Heritage status, intending to develop a cultural and tourism project emphasizing the historic, economic, and social significance of copper mining in the region. By 2007, two hotels and a restaurant (in the old American Club) should be in operation, and a new ski area  is also in the works.
Both the mine and Sewell are now open to the public for guided tours that are expanding in scope—at present, for instance, buses carry visitors into the labyrinth of underground tunnels to view a subterranean cavern of spectacular crystals uncovered in the course of extracting copper ore. Guests also view the massive ore-crusher in action, and in coming years tours will expand to include the actual mining and the smelter.
Full-day tours to El Teniente and Sewell, 2,600 meters above sea level, take place weekends only through VTS (Manuel Montt 192, Rancagua , tel. 072/210290, fax 072/211681, cell 099/7991320, www.vts.cl ); Sewell on its own costs US$22 per person; combined with a descent into El Teniente, the price is US$31. Lunch is US$5.50 extra.
Aside from the privately contracted tours, Codelco maintains a separate informative website on Sewell (www.sewell.cl ).