High in the mountains north of Termas de Cauquenes, 60 km (37 miles) northeast of Rancagua, the El Teniente Mine is the world's largest subterranean copper mine, in operation since colonial times. In 1905 the city of Sewell, known as the "city of stairs," was constructed at 2,130 meters (6,988 feet) above sea level to house miners. Abandoned in the early 1970s, Sewell was declared a UNESCO World Heritage Site in 2006. Rancagua tour operator VTS offers guided tours of both
the mine and the city every Saturday, Sunday, and public holiday, with transport from Santiago or Rancagua.