The set-up of Voyage to the Prehistoric Planet (1965) begins simple enough... The year is 2020. The moon has been colonized and now, for the first time, humans are prepared to explore Venus, because the planet is "so much like our own."
A space shuttle carrying two human astronauts and a robot named "John" lands on the surface of Venus and almost immediately loses radio contact.
Fearing the worst, a recovery team comprised of three more astronauts is launched and sets out to find them. Upon landing, they find themselves in a hostile environment. There is water and the planet can sustain life, but the existing lifeforms are those of prehistoric creatures and plants that attack the men right off the bat.
No humanoid life seems present, save for the distant ringing of a female voice and the far off glow of lights, the source of which is not immediately apparent. The men set out in their lunar car to find their friends, but come under attack by a pterodactyl over a body of water, and are forced to submerge their car.
Under water, they discover remnants of a past civilization of intelligent life. The rescue mission continues and that vague human female voice persists, forcing themselves to ask the question- "Are there still other people on this planet?"
Brief clip of Voyage to the Prehistoric Planet
It is obvious in watching this film that it was originally intended for an over seas market, and some American voices were poorly overdubbed later for marketing here. The special effects are hokey, but I like that kind of thing.