I hadn't realised this until one winters morning with a clear sky. This is exciting because Yatsugatake used to be the tallest mountain in Japan.
The story goes that Asama-sama and Gongen-sama were having an argument about which mountain was taller, Mount Fuji or Yatsugatake. Everybody knew that they were both really tall. Asama sama thought it was Yatsugatake, but Gongen sama thought it was Fuji. Gongen sama also thought that Fuji was much more beautiful.
To find out, they put a channel from the top of Yatsugatake to the top of Fuji, then put some water into it to see which way it flowed. Whichever mountain it flowed towards was surely the smaller one, and they'd be able to find out which was bigger. When they put the water in, it flowed towards Fuji, showing that Yatsugatake was taller.
This greatly upset Gongen sama. After all Fuji was so much more beautiful than Yatsugatake. So Gongen sama, in anger, caused Yatsugatake to break up into several pieces, and it wasn't taller any more. Yatsugatake's sister, Mount Tateshina, wept at his broken figure and her tears created Lake Suwa.
(Note, much of this was made up, or half-remembered, but there is definitely some kind of myth out there upon which it is based. It may have been Konohana sakuya hime...)
It's a nice story, overlooking a couple of issues with it. First of all, the logistics of building such a long channel would be significant. Not only would it curve significantly under it's own weight, but the curvature of the earth would mean that it was not straight.
Second, and more significantly, the name "Yatsugatake" means eight peaks, so called because of the rugged structure of the mountain after it was broken into pieces. So it wouldn't have had eight peaks in this story, and wouldn't have been called Yatsugatake.
Anyway, mythology aside, the story is plausible from a seismic perspective. There was very likely a volcanic eruption at Yatsugatake causing the massive mountain, which is much older than Fuji, to become smaller.
