"Man-Killing Thing of the Deepest Ocean ... where fearless men guarded a City of Gold!" - Capt. Nemo and the Underwater City One-Sheet
I saw this in 1969 while in my mid-teens and it struck me as an innocuous fantasy at the time. The monster was only briefly seen and the rest was cool sets and a message about the coming doom of man that I had already unceasingly been inundated with in the 60's by the surface dwellers. I thought PLANET OF THE APES had done it much better. No hidden message there!
Rated G, resident L.A. Times movie critic Kevin Thomas called it a "pleasant, elaborately made fantasy inspired by Jules Verne that is suitable for children and should appeal to science fiction fans".
I haven't seen this movie since as I don't think it ever enjoyed much of a re-release for home viewing. It currently can be found for sale among the MGM classic DVD-R recorded series.