A group goes down to explore the center of the earth through the opening of a volcano. Kenneth More portrays Professor Otto Lindenbrook and does his best to seem like James Mason from the 1950's film version of the book. There's also a young woman and a young man, plus their guide. Once down below they discover a huge underground ocean, that's fresh water.
There's also giant mushrooms, a giant gorilla, dimetrodons, aquatic dinosaurs, and a weird time-travel element about another explorer they encounter. It seems like the writers threw in the part with the King Kong-like monster because the DeLaurentis Kong Remake was released the year before. And a scene where two sea monsters battle to the death is quite bloody, probably influenced by JAWS. The whole weird underground city is like something out of the tv series CLIFFHANGERS. I watched this because it's the only JOURNEY TO THE CENTER OF THE EARTH movie I hadn't seen. While overall disappointing it is quirky, and very much a product of its times. Also known as "The Fabulous Journey To The Center Of The Earth".