City Beneath the Sea (1969)

This short was produced as a pilot promo for a potential futuristic underwater action adventure sci-fi show that was meant to replace Star Trek after that show went off the air. In the future, humans and humanoid amphibians live in huge futuristic cities underneath the sea, for unspecified reasons. General Matthews and his small but powerful sub Titan is approaching one of the cities, when he's informed by the Underwater Traffic Control that something is wrong with the lava experiment that's being conducted on the ocean platform on the surface. There might be a saboteur, who wants to cause an accident during the experiment and destroy the city for some reason. General Matthews' sub is the only one that can get to the platform in time and check for any irregularities.