Lake Motosu (a.k.a. Motosuko) in Yamanashi Prefecture was the filming location for the opening scenes of Godzilla vs. Megalon (1973). Photo by Brett Homenick.
In the film itself, the body of water is given the fictitious name Lake Kiriyama. In reality, the filming location was Lake Motosu (a.k.a. Motosuko), one of the Fuji Five Lakes. Situated in Yamanashi Prefecture, it stands in the shadow of Mt. Fuji, although Fuji-san is never visible in Godzilla vs. Megalon.
Mt. Fuji begins to emerge from the clouds. Photo by Brett Homenick.
Lake Motosu is rather inaccessible by rail, so travelers who wish to visit the lake should take a highway bus from Shinjuku Station. The trip is about two and a half hours one way, so it will take a large chunk of your day to see it. It's a beautiful lake, and the filming location is iconic (in my opinion, of course), but your mileage may vary.
I still think it would be cooler to ride on some plastic dolphins. Photo by Brett Homenick.
I am happy to report that there were no signs of Seatopians, and at no time did the water levels go down. I suppose Lake Motosu will be a safe haven for Godzilla fans interested in checking out locations from the coolest Godzilla films.