and in 2000, bob mckensy brought us the story. >> reporter: for most of us, what we know about this is it supplies bay area cities with drinking water. but before this, there was a beautiful place that looked so much like yosemite it was called the counter part of yosemite. it was a public treasure like its sister valley. city officials felt san francisco was paying too much and when they announced the plan to turn this in a water supply, john muir was outraged. >> no denying san francisco needs water, but you don't need to destroy a national park. >> they saw nothing wrong this and neither did san francisco's mayor. the environmentalist lost the battle and work began on the railroad that would take the supplies there. and they worked on a system to bring the water in to san francisco. it is an impressive system looked at today. this is just a portion of the eight-mile spanning. it relies on gravity, going through a ten-mile tunnel through a drop into the pipe and into the first of two power plants. the big drop turns the turbine and the water going back to make a big drop and generate