you can see the two working together at mirror lake. a parking lot allowed people to drive up to the lake, but today it is gone. it is slowing drying up because nature declares it to be so. it survives to bear out the seemingly there that to see yosemite is to love it. >> the ongoing debate about this and why it is filled with water and why some want it drained. and the battle to get ddt out of san francisco. $ >> tonight we are remembering rachel carson and john muir, whose effort was to stand up for yosemite. he failed and in 1923, a twin was transformed into a lake. 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 den