Colour film from late-1950s or 1960s. All I remember of the plot is that the protagonists are at one point captured by a strange people who live in caves, I think deep underneath the ocean, and who have developed gills so they can breathe underwater. The protagonists are imprisoned but manage to escape, or are helped by one of the gill people. At one point they are walking along a sort of causeway next to water, and a huge, black monster tall as a building bursts out of the water and stuffs one of the men in his mouth. I bet the movie had “Atlantis” in the title.
Without the gill people, the summary you wrote reminded me of the 1960 version of The Lost World. Except for the gill people and the underwater location, everything else you mentioned is in it.
I might be mixing up my memories of “The Lost World” with that of the other film. Ok, so what movies do we know that feature underground cave empires ruled by people with gills?
Maybe the 1965 movie The City Under the Sea, AKA War-Gods of the Deep.
Does look familiar. But I’m pretty sure that in my film the gill people looked like humans.
I remember a scene – my memory might be tainted by Waterworld, though, which also features an amphibian mutation – where a human reveals himself as a gill person by showing the spectators his gills, which I believe were behind his ears.
Timeframe is wrong, but perhaps you are thinking about Warlords of Atlantis
That’s it! Totally wrong timeframe, but then I was ten years old when I last saw it, and it is a low-budget B-movie. Thanks!