From the start, the plan had been to push boundaries — to go where no one had gone before, as Richard Stockton Rush III liked to say. One partner was experienced in start-ups and passionate about “making humanity a multiplanet species.” The other, Mr. Rush, was an aerospace engineer and investor with a family fortune. Both had once dreamed of going to space.

“We were frustrated astronauts,” Guillermo Sohnlein recalled. In 2009, he and Mr. Rush founded OceanGate Expeditions, the company whose submersible is presumed to have imploded during an expedition to the Titanic, killing Mr. Rush, 61, and four other people.

The founders’ goal for OceanGate, Mr. Sohnlein said, was to make deep-sea voyaging as accessible to wealthy tourists and researchers as entrepreneurs like Elon Musk had made space travel.

“Internally,” he said, “we called ourselves SpaceX for the ocean.”

As the authorities map the shattered remains of the submersible, called Titan, on the ocean floor near the Titanic shipwreck, the focus has turned to the company at the center of the expedition. The tragedy, which stunned the world and mesmerized the nation this week, came as no surprise to veteran deep-sea explorers, many of whom had warned that Titan’s novel design was a catastrophe waiting to happen and needed to undergo rigorous testing.

“When I first heard this story,” said Adam Wright, an entrepreneur in Berkeley, Calif., who spent years running a submersible company and once considered teaming up with Mr. Rush, “I was shocked that he’d gotten people to pay to go into his sub.”

In some respects, confidence was built into Mr. Rush’s DNA. He was an heir to a San Francisco oil fortune and could trace his ancestry to two signers of the Declaration of Independence. The symphony hall in San Francisco is named for his maternal grandmother, Louise Davies, who was a prominent West Coast patron of the arts.

The Apollo 12 commander Charles Conrad Jr., known as Pete, was among his family’s closest friends. As a young man, Mr. Rush had asked Mr. Conrad how he could become an astronaut, too, and was advised to consider the military and obtain a pilot’s license, Mr. Rush once said. While still in college, according to his company bio, he was flying planes in and out of Saudi Arabia during his summers at Princeton, where he earned a degree in aerospace engineering in 1984.

But his eyesight fell short of fighter pilot standards, Mr. Rush said, so he became a flight test engineer at McDonnell Douglas in the Pacific Northwest. In 1986, he married Wendy Weil, a pilot herself and a great-great-granddaughter of the retail magnate Isidor Straus, who was a co-owner of Macy’s, and his wife, Ida, both of whom died on the Titanic in 1912.

In later interviews, Mr. Rush would joke that he earned his money “the old-fashioned way” — “I was born into it and then grew it.” Affable and quick-witted, he was tapped to join the elite Bohemian Club of San Francisco, where his father was a member, and did stand-up comedy at the group’s famously secret gatherings, priding himself on never telling the same joke twice, Mr. Sohnlein said.

He had invested part of his inheritance in an assortment of tech and engineering ventures, and by the early 2000s was wealthy enough to consider traveling to space on one of the private rockets being developed.

However, while attending the triumphant Mojave Desert launch of SpaceShipOne in 2004, the first privately funded manned craft sent into space, he abruptly lost interest in being a mere passenger, he told Smithsonian magazine: “I didn’t want to go up into space as a tourist. I wanted to be Captain Kirk on the Enterprise.”

A certified scuba diver since age 14, Mr. Rush would later trace his maritime pivot to a dip in Puget Sound, which was so cold that he began exploring the possibility of securing a small submarine to more comfortably explore the water. When he discovered that very few subs were available on the private market, he said, he built a 12-foot mini-sub with a plexiglass window from blueprints created by a retired U.S. Navy submarine commander, completing it in 2006.

The project became a passion. Two years later, Mr. Sohnlein and Mr. Rush met through Graham Hawkes, an engineer and longtime submersible expert. On Thursday, Mr. Hawkes recalled Mr. Rush as a businessman intent on carving out a niche in the extreme travel market. For a time, Mr. Rush seemed interested in buying Mr. Hawkes’s submersibles company, but no deal came together.

Instead, in 2009, Mr. Rush joined forces with Mr. Sohnlein, by then a serial entrepreneur and angel investor focused on expanding human civilization to other planets.

“We figured going undersea was as close as you could get to space without leaving Earth,” Mr. Sohnlein said.

They settled on a plan to build and lease submersibles that could dive at least 4,000 meters — more than 13,000 feet — below the ocean’s surface, Mr. Sohnlein said: “Our theory was if we could take a page out of Elon Musk’s book at SpaceX and use private capital to build deep-diving subs, we could make them available to anyone who needed them — researchers, filmmakers, explorers — at a fraction of the cost.”

The company started with a used yellow submersible and shallow dives, and then graduated to a steel-hulled cylinder, Cyclops 1, that could go deeper. But the steel-hulled craft was extremely expensive to operate and transport.

Mr. Sohnlein bowed out in 2013, feeling that OceanGate had moved from the start-up phase into Mr. Rush’s specialty of engineering. Soon after he left, Mr. Sohnlein said, Mr. Rush began talking publicly about building a titanium-capped prototype of Cyclops 1 out of lightweight carbon fiber, a material common to aerospace that he felt would drastically lower the costs of operation.

Looking back, Mr. Hawkes said this week that he wished he had warned Mr. Rush that carbon fiber was too unreliable to use in the hull of a submersible, which must withstand immense pressure. One day, it could safely dive to 10,000 feet, he said, but it might suffer damage that was impossible to detect and implode the next day at 9,000 feet.

By 2017, OceanGate was advertising expeditions 12,500 feet down to the Titanic ruins in Titan, a submersible that could seat five and dive eight times as deep as Cyclops. Early news releases said tourists would pay about $105,000 apiece, a price that OceanGate set because it was the inflation-adjusted cost of a first-class ticket on the Titanic in 1912.

In the deep-sea exploration community, alarms sounded. By January 2018, OceanGate’s director of marine operations, David Lochridge, was compiling a report warning of the potential dangers to passengers.

Weeks later, several experts had a tense exchange with Mr. Rush at a conference of crewed underwater vehicle specialists in New Orleans, according to Karl Stanley, who has operated a tourist submersible in Honduras for decades. “People were basically ganging up on him in that room,” Mr. Stanley said.

Shortly after that, in March, more than three dozen industry leaders, deep-sea explorers and oceanographers warned Mr. Rush in a letter that the company’s “experimental” approach could lead to potentially “catastrophic” problems with the Titanic mission.

“We suggested, ‘Look, you’re going too fast, and the idea of bypassing the existing classification process can lead to serious consequences,’” Will Kohnen, the head of the Marine Technology Society’s committee on manned underwater vehicles, recalled on Thursday. “You don’t know what you don’t know.”

Mr. Rush, he said, threatened on a phone call to leave the industry group entirely. On its website in 2019, OceanGate said that the certification process was too slow to keep up with the company’s pace of rapid innovation. Mr. Rush had called submersibles in general “obscenely safe” and complained that the industry was too cautious.

Mr. Stanley said that in April 2019, he and Mr. Rush took the Titan on a 12,000-foot dive in the Bahamas. The vessel made such a frightening cracking noise as it plunged that Mr. Stanley implored Mr. Rush to cancel expeditions to the Titanic that already had been advertised for June.

OceanGate canceled the Titanic dive for that year, saying it had failed to secure permits for a research support vessel. But Mr. Rush pressed on. Mr. Sohnlein, who now lives in Barcelona, Spain, said critics unfairly judged Mr. Rush’s decisions.

“These people didn’t work at OceanGate, they weren’t part of the technology development program, they certainly weren’t part of the testing program and, regardless, everyone has their own opinion,” Mr. Sohnlein said. “Stockton was very risk-averse.”

The company declined to comment or answer a list of questions on Friday. “We are unable to provide any additional information at this time,” Andrew Von Kerens, an OceanGate spokesman, wrote in an email.

By 2020, OceanGate had filed documents with the Securities and Exchange Commission indicating it had raised about $18 million by selling equity to investors. Mr. Sohnlein said Mr. Rush “had probably lost money,” having kicked in on every round of investment and supplied much of the seed capital.

By 2021, after a few false starts, OceanGate was completing expeditions to the Titanic at ticket prices that had more than doubled to $250,000. Over the course of 2022, 28 people rode in Titan, according to legal documents filed by the company.

“It was a wonderful experience,” said Alan Stern, 65, a planetary scientist who was on a Titan dive to the Titanic last year. Mr. Rush was “intelligent” and “can-do.”

But, “they were frank in their paperwork and in their conversations,” he added of OceanGate. “This is not a ride at Disneyland.”

William J. Broad and Jenny Gross contributed reporting. Kitty Bennett contributed research.