This is the volcanic lake on the top of Mt. Zao, a mountain that straddles the line between Yamagata and Miyagi. I have been up to the top of this mountain and have seen the lake, which is quite beautiful.
As you can see from this postcard which I sent recently, the colour is quite unusual.
I do have a spare copy of the Gotochi, so you never know who is going to end up with it!