El Teniente is the world's largest underground copper mine — a vast network of tunnels and chambers honeycombing a single mountain in the Andes east of Rancagua. The mine has been worked, in one form or another, for more than two hundred years, and is the reason Rancagua exists at its modern scale.
Tours go in via Sewell, generally as part of a single-day package with VTS Tours and a few others. The route descends into a chamber at the upper level — visitors don boots and helmets, ride a bus into a side tunnel, and walk a hundred metres or so into the rock. It is not a deep-mine experience, but the scale of the operation is impressive. Independent visits are not possible; everything is on guided tours from Rancagua.