What was the movie where there was an underwater UFO base? I think it was a feature-length film, in color, as I definitely saw it a theater in the mid to late '70s. Don't remember if it was Japanese or not.
I think the UFOs were classic saucers, without any big periscope or any attachments like the War of the Worlds had. In one scene, a UFO is flying along the ocean, and then decends and splashes right into the water! Then it is coasting along undersea, and enters through a rocky doorway or cave? Maybe?
Thanks to the people who have responded already, still not what I remember. Darn I wish I could remember more.