Warlords of Atlantis (1978)

There were a quartet of sci-fi adventure films in the 2nd half of the seventies which took place in an earlier era, of which this was the last one. The previous ones were The Land That Time Forgot (75), At the Earth's Core (76) and The People That Time Forgot (77). These also all featured Doug McClure, usually in the lead role, as in this one: he plays an engineer in the year 1898; he and a bumbling colleague (Peter Gilmore) are heading into the Bermuda Triangle on the rigger Texas Rose. Then, going beneath the waves in a revolutionary Bathysphere, they are accidentally swept into a cavernous world beneath the seabed and end up in the fabled City of Atlantis.  McClure also has to deal with mutineers and, of course, strange creatures. The big sci-fi element here is the backstory created for these Atlanteans, who it is revealed came over here inside an asteroid from their dying planet thousands of years ago. Their society is also eventually revealed as not that benevolent and involves slavery and forced mutation to live underwater. The difference between this and the previous films is that the earlier ones were based on works by Edgar Rice Burroughs, while this was an original story by Brian Hayles.