The five people aboard the Titanic-bound submersible lost under the sea are likely dealing with not only a frightening experience but an uncomfortable one as well, according to one expert.

“From what I understand, the vessels are not designed for long-range, multi-day excursions,” John Mixson, a retired US Coast Guard lieutenant commander told Fox News.

“So it’s going to be a very, very uncomfortable, dark experience with a lot of hope and prayers,” Mixson said.

The watercraft, operated by OceanGate Expeditions, was reported missing after it failed to return to the Canadian research ship from which it was launched Sunday morning and crews lost contact with its captain.

“It’s hard to say whenever you just lose total communications in a situation like that what actually happened until you find the vessel,” Mixson told Fox News. “This isn’t a common occurrence at all.”

“Obviously, something very rapid and very tragic took place,” he added.

Search and rescue teams are frantically looking for the lost vessel as the remaining number of hours of life-sustaining oxygen levels inside the sub ticks down lower and lower from 96 hours at 6 a.m. Sunday.

“I would say it is extremely serious. It’s a dire situation,” Mixson said. “But on the other side of that fact, it is still considered classified as a search and rescue mission, which should give everyone hope, including the family members and friends of the people on board the vessel.”

One of the craft’s passengers is British businessman and billionaire adventurer Hamish Harding. The identities of the four others aboard have not been released.

The US Coast Guard is leading the search effort for the submersible — which was headed to the famous underwater Titanic wreck as part of a research and tourist excursion. OceanGate Expeditions, the private company that launched the vessel, offers private tours of the wreckage for as much as $250,000 a head.

The search is focused on an area about 900 miles off the coast of Cape Cod at a depth of roughly 13,000 feet — making it the deepest-ever rescue mission in history if successful.

The record-breaking depth and remoteness of the location are making the search particularly challenging, Coast Guard officials and experts said.

“If [the submersible] has gone down to the seabed and can’t get back up under its own power, options are very limited,” said Alistair Greig, a professor of marine engineering at University College London. “While the submersible might still be intact, if it is beyond the continental shelf, there are very few vessels that can get that deep, and certainly not divers.”

Both aircraft and ships are involved in the search for the vessel.

Authorities are also working to get a remotely operated vehicle that can plunge to depths of up to 20,000 feet to the site of the missing sub, according to an advisor to OceanGate, David Concannon.