In the fourth season of "The Twilight Zone", they experimented by making the show an hour long instead of the usual half hour. In some cases the new episodes worked well and in other cases it really made you look back fondly at the original format. Obviously the experiment was not successful, as the next season returned to the original length. If you would ask me why I think the shows ultimately failed, I think it's because too often they were just too long--drawing out a half hour story too far--thus ruining the suspense. "The Thirty-Fathom Grave" is a prime example of a story that was harmed by this.
The episode is set on a modern US Navy ship. When they are in the middle of very routine duty, they suddenly begin hearing tapping sounds on the sonar--very much like the sound you'd hear from a crew on a wrecked sub as they tap on the ship as a distress call. The ship stops to investigate and the next 40 minutes are basically doing nothing--waiting until the ultimate twist occurs. But, because it took so long, you are left feeling disappointed.
It's a shame, really, as one of the stars is a fun actor from the era--Simon Oakland. I loved seeing him on a wide variety of shows playing gruff blow-hard characters--especially since here he is NOT that sort of fellow. Because the show was a failure, I am sure his fine performance was overshadowed by the shallow plot and overacting of the main character--who was written in a rather bizarre and inexplicable manner.
Perhaps it's worth seeing, but it is far from a classic despite some reviews giving it a 10. In fact, EVERY episode of the series has a few people giving it 10s...making you wonder if either guys like me are idiots because we don't love every episode or there are people who are such die-hard fans that you need to take these glowing reviews with a grain of salt!