After a massive earthquake severs all communication, a scientist in an undersea lab must come to terms with one of two terrifying facts, that he is losing his sanity or that he is not alone.
it’s not bad.
I see it like a little sci-fi/horror movie like “moon” where a lonely man on a space station starts hallucinating.
To increase the potential of the logline you could give some details about what kind of ‘presence’ is haunting the lab and maybe you can try to resolve the end of the logline in a less teasing way.
I suggest to make the logline shorter avoiding “massive”.
Describe the main character with one adjective (well chosen), stress the irony or his flow or whatever make sense with the plot. Is him a widow like the solitary hallucinating astrounaut of “solaris”?