Aka “Warlords of the Deep”, though I still prefer the Atlantis connection, this 1978 film is everything you could possibly hope for in a lost world adventure. Aside from it’s somewhat repetitive soundtrack, there’s beautiful matte paintings, quality practical effects, well done (sorta) miniatures, and some actually good actor to projection screen scenes. The main creatures were called millipedes by the characters, but looked exactly like mudskipper fish to me, which I find to be adorable little guys. The creatures in the movie were 10 stories tall though, so somewhat less adorable.
I’m a fan of the story they told here, in which the Atlanteans are actually homesick Martians that are waiting for humanity to build it’s tech level to a level that would allow them to rebuilding their Martian spaceships and go home. At least, I THINK that’s what they were wanting to do, it got really convoluted there for a moment, but ultimately was a fun ride.