The submersible, operated by the private company OceanGate as a high-priced adventure travel opportunity, was supposed to visit the Titanic, which rests on the muddy seafloor 12,500 feet below the surface of the Atlantic. But on Sunday, the submersible endured some kind of incident or malfunction and went silent 1 hour 45 minutes into its 2½-hour dive.

Missing Titanic submersible: Live updates on Titan search amid fears of oxygen supply

The very deep sea is a forbidding, almost alien environment, inhabited only by odd, eyeless creatures that have adapted to pressures that could instantly crush the most advanced Navy submarine. What’s more, the Titanic’s resting place is 370 miles from the Canadian coast, and farther than that from any port from which rescue vessels can be deployed.

These challenges made the search for the missing vessel and its five occupants akin to an effort to rescue astronauts, retired Adm. Thad Allen, former commandant of the Coast Guard, told The Washington Post.

“This is closer to Apollo 13 than a classic search-and-rescue mission,” Allen said. “Trying to extract a vessel from 12,000 feet is extraordinarily difficult, if not impossible.”

The Titanic sits beyond United States territorial waters, but the U.S. Coast Guard, under a treaty governing ocean rescues, led the effort to find the submersible. Coast Guard officials on Wednesday continued to describe their work as a search-and-rescue mission even as they warned that the occupants of the vessel, if still alive, would be expected to run out of oxygen early Thursday.

The Coast Guard announced late Thursday morning that a remotely operated vehicle had found a “debris field” near the Titanic and that it was being evaluated. Thursday afternoon brought the grim news from the Coast Guard that the five-person crew had been killed in a catastrophic implosion of the Titan’s pressure chamber.

The water pressure where the Titanic rests is about 6,000 pounds per square inch. By comparison, atmospheric pressure is 14.7 pounds per square inch. Pressure builds in a linear fashion as a submersible descends. At the time the Titan submersible went silent, it would have been dealing with pressures hundreds of times greater than at the surface.

To offer one visual image: The average depth of the planet’s seafloor hydrothermal vents is 7,000 feet. Researchers studying these ecosystems — where life exists using chemosynthesis rather than photosynthesis — have carried standard foam cups to such depths. The cups retain their form but are compressed to the size of a shot glass.

“I think we all agree that going somewhere that has 400 times the pressure in the atmosphere is a dangerous thing to be doing,” said Jules Jaffe, research oceanographer emeritus at Scripps Institution of Oceanography, part of the University of California at San Diego.

But he supports exploration of this kind and pointed out that another private company, Triton Submarines, has successfully ferried tourists to the bottom of the Mariana Trench, the deepest spot in any ocean, nearly seven miles below the surface.

“Obviously [the Titan submersible] worked for a while, and they had some number of successful dives. But as they continued to use it, there is a chance that the mechanical properties became fatigued,” Jaffe said.

Lisa Levin is a veteran oceanographer and deep-sea biologist at Scripps who has plunged into that strange world many times. She studies the bizarre chemosynthetic organisms feeding off methane leaking from the seafloor along continental margins. She has made 53 dives on the famed Alvin submersible that has been in operation, with many upgrades and inspections, since the 1960s.

She once was at 1,500 meters (almost 5,000 feet) on an undersea mountain off the coast of Mexico when everything went dark, inside and outside. The pilot fiddled with electrical wiring and couldn’t get it lit up again, and so they quickly resurfaced.

She said she does not like talking about things that can go wrong. But in the case of the Titan, she said Wednesday, she feared that something might have happened suddenly and catastrophically, such as an implosion. Had there been a gradual leak, the mission would have been quickly aborted. Getting snagged in a net or otherwise stuck somewhere should not have ended communication with the expedition ship at the surface, she said.

“If anything’s not working properly, they abort the dive. It’s like an airplane — if something’s not quite right, they’ll land at the nearest airport,” Levin said.

“If anyone had seen a leak, they would have dropped weights and come to the surface,” she said. “I’m guessing whatever happened, happened quickly.”

Many submersibles have ventured into the deepest parts of the sea. James Cameron, the director of “Titanic,” is among those who have managed a close-up look at the doomed ocean liner via submersible. Cameron has made that same dive dozens of times, as well as a 2012 dive to the Mariana Trench.

But risk does not evaporate with each successful venture. Engineers know that risk builds like plaque, and complex technology deployed in harsh environments — such as space or the deep sea — can fail in many ways not easily envisioned.

The NASA space shuttle program carried with it known risks of disaster. The Challenger met a catastrophic end just minutes after launch when a component of a solid rocket booster failed, leading to the ignition of the external fuel tank. The Columbia was destroyed by the heat of atmospheric reentry penetrating the leading edge of a wing damaged at launch.

Levin, the Scripps oceanographer, said finding a lost submersible on the seafloor would be a tremendous challenge. The deep sea remains a frontier. It is less well mapped, scientists say, than the surface of the moon.

“It’s like looking for a small plane that goes down in the middle of a giant rainforest. It’s just hard to see,” she said. “We don’t have easy ways to image the bottom and distinguish the wreck of a submarine from anything else on the bottom.”

The Titan submersible was a prototype craft, and the OceanGate journeys are a form of adventure travel that exists beyond the reach of government regulatory agencies, said Allen, formerly of the Coast Guard. This is not a situation where agencies have vessels standing by, ready to conduct rescue operations on choppy seas in hurricane season in the middle of the Atlantic Ocean.

“We’re dealing with the high seas and the depths of the ocean — they’re one of the few ungoverned places on Earth,” said Allen, who knows a thing or two about offshore disasters, having served as the no-nonsense national incident commander for the response to the 2010 BP oil spill.

Typically the depths are reached with robotic ROVs — remotely operated vehicles. But ROVs are not like firetrucks with their noses facing the street down at the fire station. They are specialty hardware and not easily deployed on a moment’s notice to plunge into mid-oceanic waters to search for a missing 22-foot submersible.

The Coast Guard reported Thursday morning that the Horizon Arctic, a Canadian vessel, had successfully deployed an ROV to the seafloor to begin its search for the missing submersible. Soon thereafter, the French vessel L’Atalante also deployed an ROV.

The depths of the Atlantic can’t be reached by human divers without specially designed submersibles. The creatures of the deep are adapted to the pressure, the cold and sometimes other extreme conditions, such as low oxygen levels or high levels of hydrogen sulfide. Levin, the veteran deep ocean explorer, points out that humans never experience those conditions directly.

“As a scientist, diving in a submarine, other than being a little bit cold, we don’t experience the high pressure and the other challenges in the water,” she said. “We build submarines to keep the internal environment comfortable for us.”

And they trust the technology to work — knowing that there may come a moment when it doesn’t.