?Writer and former showrunner on the popular sitcom 'The Simpsons' Mike Reiss says he's "not optimistic" about a rescue of the missing submarine that was exploring the Titanic wreckage.

He has an informed opinion - it's a vehicle he has taken several trips on prior to this incident, reports deadline.com.

The submersible 'Titan' vehicle has been missing since June 18 in the North Atlantic, with five people on board.

"I know the logistics of it and I know how vast the ocean is and how very tiny this craft is," he told BBC Breakfast on June 19.

"If it's down at the bottom, I don't know how anyone is going to be able to access it, much less bring it back up," he said.

Reiss has gone on three different dives with OceanGate Expeditions, the company which owns Titan, and that they "almost always lost communication".

"I got on the sub and at the back of my mind was 'well, I may never get off this thing,' that's always with you,' he said.

While Reiss agrees that Titan is "a beautifully designed craft."

"This is not to say this is a shoddy ship or anything, it's just that this is all new technology and they're learning it as they go along," he said.

"You have to just remember the early days of the space programme or the early days of aviation, where you just make a lot of mistakes on the way to figuring out what you're doing."

The Titan is expected to run out of its emergency oxygen supply around 10.30 a.m. GMT on Thursday.

Another entertainment industry figure also was acquainted with the Titan.

Josh Gates, host of Discovery Channel's Expedition Unknown, tested out the Titan for a possible segment for his show. He told Anderson Cooper that the vessel was "extremely claustrophobic."

Gates and his team didn't use the vehicle because of other concerns.

"On one hand, you have this incredibly innovative, novel design, on the other hand, there are a lot of unknowns," Gates said.