(NewsNation) — The missing submersible carrying a group of people touring the Titanic wreckage was “constantly plagued by mechanical problems,” a person who was onboard the OceanGate submersible last year said.

David Pogue is the host of the “Unsung Science” podcast and is also a “CBS Sunday Morning” correspondent. He told “NewsNation Now” last year he was on the same submersibles that is now missing for a story.

“I never got to see the Titanic,” he explained.

Pogue said they got about 37 feet down when they ran into mechanical problems and had to return to the surface.

“But, that’s typical,” Pogue said. “They take these millionaires out there for five days at a time, so they have five chances to see the Titanic. Most weeks they make it only once or this summer, not at all.”

Pogue told NewsNation the missing submersible Titan is “homemade” by OceanGate Expeditions CEO Stockton Rush.

“It’s about the size of a minivan. It seats five people. There’s no chairs in there, you’re just squatting on the floor,” Pogue said. “The journey is about 10 to 12 hours to go down to the Titanic, find it, tour it and then come back up to the surface ship. It is constantly plagued by mechanical problems.”

Pogue says the problems aren’t unique to OceanGate.

“This is all of these Titanic subs. There’s only about five of them in the world, and they all have these kinds of mechanical malfunctions, electrical malfunctions,” he said.

Pogue said the Titan has around seven different ways to return to the surface.

“There’s backups of backups of backups. If it’s not at the surface, then only one of two things could have happened. Either it’s snagged on something like a big fishing net or even part of the Titanic, or there was a leak, in which case it would implode instantly and it’s over,” Pogue said.

“If you want to look optimistically at it, they lost power but they have three more days of air and they’re bobbing on the surface somewhere, and we just can’t find them because they don’t have power.”

The Coast Guard said there is one pilot and four “mission specialists” aboard the missing vessel but did not provide their identities. The agency is working with Canadian authorities to search the surface, about 900 miles off the coast of Cape Cod, by using aircraft and ships and sonar buoys in the water.

Pogue said there’s only three operating submarines in the world that can go to the depth of the Titanic, so finding the submersibles is not the only challenge. Bringing the missing submarine to surface and finding a way to let the people inside escape, he says, are also major obstacles.

“The hatch, by the way, is sealed with 17 bolts from the outside. So there’s no way to escape when you’re inside, to get out without a crew on the outside letting you out,” Pogue said.

In the event of an emergency, the Titan is designed for passengers to be able to survive for 96 hours, the Coast Guard said. It is unknown whether there was an issue on the submersibles or if it is lost.

OceanGate said they are “exploring and mobilizing all options to bring the crew back safely.”

“Our entire focus is on the crewmembers in the submersible and their families,” OceanGate Expeditions said on Twitter. “We are deeply thankful for the extensive assistance we have received from several government agencies and deep sea companies in our efforts to reestablish contact with the submersible.”

The vessel takes tourists, paying anywhere from $100,000 to $150,000 each, to view the remains of the Titanic, which are about 12,000 feet at the bottom of the Atlantic Ocean off the coast of Newfoundland, Canada. The Titanic sank in 1912 after hitting an iceberg.