Ran into two old sci-fi films I've heard of, but never have watched. Both ended ambiguously, which is something I don't mind.
The Quiet Earth (New Zealand, 1985). A well done, in my opinion, 'last man on earth' film. There were two other survivors. Not sure at all where the ending took the original guy.
The Day the Earth Caught Fire (1961). Reminded me of a Twilight Zone episode called The Midnight Sun (1961) except the film didn't have a trick ending where everything was actually freezing. The film ended before we discovered if a whopping huge correction effort succeeded. Now, the version I saw was the American version where an audible sound at the end was added to make folk think the effort succeeded. All other countries versions didn't have the audio cue, leaving it ambiguous.