In the late 1960s I discovered the Lincoln Arts Cinema on West 57th Street in Manhattan - my sister and I went there to see THE LION IN WINTER when it came out. Subsequently I went by myself or with others, in particular to see the series of re-released films of Charlie Chaplin in the early 1970s. But all things are ephemeral, especially buildings and businesses. The Lincoln Arts Cinema eventually went out of business. One of the last productions I saw there was a double bill (a rarity) of THE NEPTUNE FACTOR with a comedy (possibly RETURN OF THE PINK PANTHER, but I am not sure). It was a sign of the times that this showcase for single film classics or would-be classics ended up doing a double bill. Also that one of the films was THE NEPTUNE FACTOR.
I rarely went to science fiction movies, so it must have been the comedy that I really wanted to see. In any event, for some reason this "inner space" story lingered in my memory rather than the other film (even if it was the Sellers' movie). For one thing it intrigued me to watch a movie about underwater exploration (deep sea at that). The period was one when there were too many submarine disasters: the "Thresher", the "Scorpion", the "Dakar", the "Minerve". So there was a realization of the potential for total disaster as we explored or traveled in the seas. The real achievements in underwater exploration were still in the future (it was another decade before Dr. Robert Ballard located the wreck of the "Titanic"). So the potential for an interesting film was there. Regrettably the movie was not that good - but it does make one want better films on the subject.
At some near future date there is an underwater station set up on the ocean floor. An earthquake occurs, and the crew is trapped. So the government sends a team with a submarine to do a difficult rescue. The team is headed by Walter Pidgeon and Yvette Mimieux, and it sends the submarine under Ben Gazzara and Ernest Borgnine to the rescue. What follows in the film is a kind of special effects travelogue of what the rescue crew sees under water, and how they overcome various obstacles until they finally locate the toppled tower of the underwater lab - just in time (of course) as some monster of the deep is attacking (I recall it does get one of the endangered crew, who is fighting it to save his comrades).
The photography was the best part of the movie - it did make the viewer take interest in that portion of the globe that covers three fifths of our planet. But as is rightly pointed here on the thread it did not really show the bizarre lifeforms of the deep sea - creatures that rarely see sunlight. Of course, the real problem is that deep sea photography (before Dr. Ballard's descents, two and a half miles down) were extremely difficult - and impossible for a commercial Hollywood film. So one has to allow for this failure while enjoying the fish and fauna that is seen.
The performers try hard, particularly Borgnine, who attempts some comic relief. A reviewer on this thread mentions Gazzara's performance as wooden. I hesitate to go that far (he seemed more tired than wooden), but I have seen him to far better effect in THE STRANGE ONE and other performances. Pidgeon was in his film dotage - trotted out as an authority figure of one type or another at the time, with his aged appearance erasing memories of the good looks of thirty years before in MRS. MINIFER or HOW GREEN WAS MY VALLEY. In his later years every time I saw Walter Pidgeon's performances (unless the film script gave him some good moments) his thickened pursed lips always made me think him a caricature of himself.
In the end, the interest in the subject matter is the best thing going for THE NEPTUNE FACTOR, coupled with the photography (even with it's limitations). So I give it a "5" out of a possible "10".
Review by theowinthrop from the Internet Movie Database.