Season four of Twilight Zone is the only one of the five seasons that ran its episodes in hour long time slots rather then the conventional half-hour format. The Thirty-Fathom Grave was written by Rod Serling before the network and producers decided to try out the series in the new lengthier format. Since the episode had already been optioned for season four it was necessary for Rod Serling to re-write and expand the episode to fit the new hour slot. Therefore several new scenes had to be added or padded to fill up time. As a result, this episode received mostly negative feedback based on its slow pace and unnecessary dialogue.
The exterior shots of the ship used in this episode were of the The "Mighty Mux" USS MULLINNIX DD-944, A Forrest Sherman class destroyer. The interior shots were done on board the USS-Edson DD946.
The position of the ship and sub is given as 09°30' N 160°48' E, which places them about two miles north of the island of Nura in the Solomon Islands, roughly in the middle of the Solomon Sea.
Mike Kellin portraying the main character, Chief Bell, died 26 August 1983, the ship used for exterior shots in the episode was decommissioned 11 August 1983. Simon Oakland, who portrayed the captain, died three days later on 29 August 1983.
According the dialogue, the sub was reported lost on August 7th, 1942 which coincides with the Allied landings in Guadalcanal, Tulagi, and the Florida Islands in the Eastern Solomon Islands chain, igniting what we know today as the Solomon Islands campaign. However, no American submarines were present in the area on August 7th. In fact, according to the USN order of battle for the campaign, no American submarines participated at all.