What's it really like 12,500 feet below the surface?

A passenger who previously rode on the missing Titanic research submersible is here to explain.

As Aaron Newman—an investor in OceanGate who toured the wreckage 350 miles off the coast of Canada in 2021—put it, the expedition as "basically going to another planet."

"You're getting in this craft—you're bolted in," he told Today on June 21. "It's a tube that's comfortable, but not spacious."

In the beginning, the submersible's cabin is "very hot and stuffy," but Newman said the water surrounding the vessel drops to 29 to 30 degrees Fahrenheit as it submerges deeper into the ocean.

"By the time you hit the bottom, the water down there is below what standard freezing temperature is," he explained. "That's going to conduct right through that metal, so it was cold when we were at the bottom. You had to layer up—we had wool hats on and were doing everything to stay warm at that bottom."

And though Newman felt "very safe" while riding in the sub, he noted "there are risks" in taking such a challenging expedition. 

"This is not a Disney ride, right?" he remarked. "We're going places that very few people have been, and this is inventing things."

Newman added that the five passengers on the sub, missing since June 18, must have accepted these risks prior to their mission. 

"None of these people were people that were I would consider tourists—tourists is such a bad term," he said. "These are people who lived on the edge and loved what they were doing and if anything's going on, these are people that are that are calm and thinking this through and doing what they can to stay alive. So this is a good set of people."

Since the 21-foot sub went missing, rescuers have been working around the clock to locate the watercraft, which officials estimate has less than 40 hours of oxygen left in its cabin.

Among the missing is British billionaire Hamish Harding and OceanGate CEO Stockton Rush.

"Our focus is just hoping for this Hollywood ending to happen," Newman said of the missing sub and its crew. "We know the Coast Guard and everybody else is working so hard. And the OceanGate crew is working as hard as they can to possibly find this if anything is out there."

He added, "It's promising but there's work to be done, and that's what the focus is."

Keep reading for more details on the missing passengers.

On June 18, 2023, a deep-sea submersible Titan, operated by the U.S.-based company OceanGate Expeditions and carrying five people on a voyage to the wreck of the Titanic, was declared missing. Following a five-day search, the U.S. Coast Guard announced at a June 22 press conference that the vessel suffered a "catastrophic implosion" that killed all five passengers on board.

Pakistani-born businessman Shahzada Dawood and his 19-year-old son Suleman Dawood, both British citizens, were also among the victims.

Their family is one of the wealthiest in Pakistan, with Shahzada Dawood serving as the vice chairman of Engro Corporation, per The New York Times. His son was studying at the University of Strathclyde in Glasgow, Scotland.

Shahzada's sister Azmeh Dawood told NBC News that Suleman had expressed reluctance about going on the voyage, informing a relative that he "wasn't very up for it" and felt "terrified" about the trip to explore the wreckage of the Titanic, but ultimately went to please his father, a Titanic fan, for Father's Day.

The Dawood Foundation mourned their deaths in a statement to the website, saying, "It is with profound grief that we announce the passing of Shahzada and Suleman Dawood. Our beloved sons were aboard OceanGagte's Titan submersible that perished underwater. Please continue to keep the departed souls and our family in your prayers during this difficult period of mourning."

OceanGate CEO Stockton Rush was the pilot of the Titan. The entrepreneur—who founded the research company in 2009 in Everett, Wash.—had long been interested in exploration. Rush, 61, previously said he dreamed of becoming the first person on Mars and once said that he'd "like to be remembered as an innovator."

In addition to leading voyages to see the remnants of the Titanic, Rush had another surprising connection to the historic 1912 event: His wife Wendy Rush is the great-great-granddaughter of a couple who died on the Titanic, Ida and Isidor Straus.

British billionaire Hamish Harding confirmed he was a part of the mission in a June 17 Instagram post, a day before the submersible went into the water and disappeared.

"I am proud to finally announce that I joined @oceangateexped for their RMS TITANIC Mission as a mission specialist on the sub going down to the Titanic," he wrote. "Due to the worst winter in Newfoundland in 40 years, this mission is likely to be the first and only manned mission to the Titanic in 2023. A weather window has just opened up and we are going to attempt a dive tomorrow."

Harding—the chairman of aircraft company Action Aviation—said the group had started steaming from St. Johns, Newfoundland, Canada and was planning to start dive operations around 4 a.m. on June 18. The 58-year-old added, "Until then we have a lot of preparations and briefings to do."

His past explorations included traveling to the deepest part of the ocean in the Mariana Trench, telling Gulf News in 2021, "It was an incredibly hostile environment. To travel to parts of the Challenger Deep where no human had ever been before was truly remarkable."

The Dubai-based businessman also circumnavigated the Earth by plane with the One More Orbit project and, last year, took a trip to space on Amazon founder Jeff Bezos' Blue Origin New Shepard rocket. Harding shared his love for adventure with his son Giles, described as a "teen explorer" on his Instagram.

As for the fifth member, a representative for French explorer Paul-Henri Nargeolet told the New York Times that he was a passenger on the Titan, with Harding also referencing him on Instagram as a member of the team. 

The Times described him as a maritime expert who was previously part of the French Navy. The 71-year-old was a bonafide Titanic specialist and has traveled to the wreckage 35 times before. Nargeolet served as the director of RMS Titanic, Inc., a company that researches, salvages and displays artifacts from the famed ship, per the outlet. 

Alongside fellow passenger Hamish Harding, he was a member of The Explorers Club, founded in 1904.

As Harding noted in his post, the submersible—named Titan—was a part of an OceanGate Expeditions tour that explores the wreckage of the RMS Titanic, which infamously sank in 1912.

The company expressed its sympathies to the families of the victims. "These men were true explorers who shared a distinct spirit of adventure, and a deep passion for exploring and protecting the world's oceans," OceanGate said in a statement. "Our hearts are with these five souls and every member of their families during this tragic time. We grieve the loss of life and joy they brought to everyone they knew."

(E! and Today are both part of the NBCUniversal family.)

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