In my opinion, the two most under-rated Scifi films are
“City Beneath the Sea”, with Stuart Whitman. Set in the future, it used some of the props from “Voyage to the Bottom of the Sea”, including the Flying Sub.
Doppleganger, called “Journey to the Far Side of the Sun” in the US.
Yeah, I recall “Journey to the Far Side of the Sun”. It was pretty well done.
Another one that is cheesy, but notable, was “Battle Beneath the Earth”. Basically, the Red Chinese Army was tunneling under the US and planting atom bombs under American cities. Just what Hamas wishes it could do to Israel.