The Titan had one toilet and no seats; passengers sit cross-legged on the floor.
HALIFAX, Nova Scotia -- A submersible on a tour of the wreckage of the Titanic was reported overdue by OceanGate Inc. on Sunday, prompting a Coast Guard search effort for the 22-foot, 23,000-pound vessel.
The submersible had a four-day oxygen supply when it put to sea around 6 a.m. Sunday, according to David Concannon, an adviser to OceanGate Expeditions, which oversaw the mission.
Stockton Rush founded Washington-based OceanGate Inc. in 2009 to make deep-ocean exploration more accessible to scientists and tourists.
Fourteen years, more than 200 dives and three submersible designs later, the company now finds itself in a desperate search to recover the submersible carrying five people aboard that's gone missing off the coast of Newfoundland, Canada.
OceanGate has operated three submersibles since its inception, with the first two vessels -- the Antipodes and Cyclops 1 -- able to reach 1,000 feet and 1,640 feet, respectively.
OceanGate's only submersible able to reach the Titanic's wreckage is called the Titan, a carbon-fiber and titanium vessel that can reach 13,123 feet, the company says on its website. (The Titanic rests 12,400 feet under the surface).
"It's not a ride at Disney, you know," OceanGate Expeditions software security expert Aaron Newman said in a promotional video for the expedition. "There's a lot of real risk involved, and there's a lot of challenges."
The vessel is primarily constructed of lightweight carbon fiber, which is spun into a rigid tube for the vessel's body, specification sheets show. Two titanium caps are secured to the carbon-fiber body, with one cap including a thick transparent porthole.
Nine feet wide and only 8 feet tall, the Titan leaves little room for its passengers, who sit on a subfloor inside the carbon-fiber tube. A photo in the Titan's specification sheet shows five passengers seated on the vessel's floor with limited room to move or stand.
There is only one toilet, and no seats; passengers sit cross legged on the floor. There are no windows except the porthole through which passengers view the Titanic.
It's a "tiny vessel, quite cramped and small," said CNN correspondent Gabe Cohen, who sat in Titan in 2018 while reporting on OceanGate Expeditions for CNN affiliate KOMO. "You have to sit inside of it, shoes off."
In case of an emergency, the submersible is equipped with basic emergency medical supplies and pilots have basic first aid training, according to OceanGate Expeditions' website.
Aaron Newman, who has been a passenger on the Titan, told NBC's "Today" show Wednesday that if the submersible is below a couple hundred meters and without power, the passengers are in complete darkness and it's cold.
"It was cold when we were at the bottom," he said. "You had layered up. You had wool hats on and were doing everything to stay warm at the bottom."
He said describing the missing passengers as "tourists" is a misnomer.
"These are people who lived on the edge and loved what they were doing. If anything's going on, these are people that are calm and thinking this through and doing what they can to stay alive," Newman said, adding that he felt safe and in the hands of professionals on his descent. "It's a good set of people."
John "Danny" Olivas, a retired astronaut who has completed two stays on NASA's underwater habitat and trained on underwater spacewalks, told CNN that the expedition is "a very stressful situation."
"There probably wouldn't be any cabin air circulation, which could also pose a lot of potential hazards with just breathing the air. The oxygen is important, but also CO2 generation by five people in a small, confined vessel is going to be very challenging and potentially creating a poisonous environment for the crew members," Olivas told CNN's Victor Blackwell.
Mike Reiss, who has done four, 10-hour dives with OceanGate, including one to the Titanic, told ABC News his sub lost contact with the host ship on every dive.
"Every time they lost communication -- that seems to be just something baked into the system," he said.
With no GPS, Reiss said it took his crew three hours to find the Titanic despite landing just 500 yards from the ship.
Reiss said he signed "a waiver that mentions death three times on the first page."
"It is always in the back of your head that this is dangerous, and any small problem will turn into a major catastrophe," he said.
He said the submersible is built simply and is "just propelled by two fans on the outside."
"Even I was able to steer and navigate the sub for a while," he noted.
Reiss said his greatest fear was that the sub wouldn't be able to release the weights that force it to submerge once it was time to rise to the surface.
The Associated Press and CNN Wire contributed to this report.