Missing Titanic sub has 'very limited' options for rescue, experts say

Nine hundred miles east of Cape Cod, and 12,500 feet down, lie the remains of the Titanic shipwreck. It has become a high-end tourist attraction and the destination of a private submersible vehicle with five people onboard, which is now missing with a limited supply of oxygen.

The super strong, titanium and filament wound carbon fiber submersible, called Titan, was designed and rated to dive to just over 13,000 feet.

It had a four-day oxygen supply when it went to sea around 6 a.m. Sunday, according to David Concannon, an adviser to OceanGate Expeditions, which oversaw the mission.

The vehicle uses two communication systems — text messages that go back and forth to a surface ship and safety pings that are emitted every 15 minutes to indicate that the sub is still working.

Both of those systems stopped about an hour and 45 minutes after the Titan submerged.

"It surprises me that they lost communication so early in the dive. It takes about two hours to reach the wreck. So whatever happened, happened either on the way down or as soon as they got there," said Titanic historian Don Lynch.

The environment that far under the ocean surface is unforgiving.

Just 650 down, the sea becomes a lightless pitch black abyss. Contact was estimated to be lost at about 8,700 feet.

At that depth, the water temperature is lower 30 degrees Fahrenheit. Experts say it would be like sitting in a refrigerator since the submersible has no heating.

The massive, relentless water pressure is at almost two tons per square inch, and it builds to three tons on the sea floor at the Titanic's resting place.

Naval architect Jim Antrim, who designs ships and boats described a way to picture those types of depths: "[It's] one and a half times the height of the Salesforce Tower. That's the pressure of water down at the depth of the Titanic."

Any defects, cracks, or leaks could cause the Titan to implode in a nanosecond.

The submersible had seven backup systems to return to the surface, including sandbags and lead pipes that drop off and an inflatable balloon. But it could not do that if the extreme pressure collapsed its hull or if the vehicle was trapped.

"It's first problem is that it's probably somewhere attached to a fishing net, some debris, it may have simply gotten caught," said Butch Hendrick, a rescue diver and founder of Lifeguard Systems, who has been teaching underwater rescue methods for more than 30 years.

Eric Fusil, director of the University of Adelaide's Shipbuilding Hub, said there are other scenarios that could cut communications, including an electrical fire that could create toxic fumes and render the crew unconscious.

Another possibility is that the submersible became entangled in the wreck of the Titanic and is stuck there, Fusil said.

"What I would like to believe ... is that Titan suffered from a power loss, but they could still go back to the surface" and be spotted by aircraft and ships," he said.

But some experts say retrieval is nearly impossible.

Navy rescue sub missions are limited to 2,000 feet depth. Specialized subs can dive to a maximum of about 4,200 feet.

Alistair Greig, a professor of marine engineering at University College London, said "options are very limited" for rescue.

"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," Greig said.

Another ultra deep remote vehicle could be used to latch onto the Titan if the missing sub is found. But there are only a few vehicles in the world that can even reach those depths, and mobilizing them are not necessarily quick tasks.

With a search area of some 10,000 square miles, a multinational task force of searchers and rescuers are racing against the clock to find and save the five people onboard.

"This is a complex search effort which requires multiple agencies with subject matter, expertise and specialized equipment," said U.S. Coast Guard Captain Jamie Fredrick.

The Canadian research icebreaker Polar Prince, which was supporting the Titan, continues conducting surface searches with help from a Canadian Boeing P-8 Poseidon reconnaissance aircraft, the Coast Guard said on Twitter.

Two U.S. Lockheed C-130 Hercules aircraft also conducted overflights. The Canadian military dropped sonar buoys to listen for any sounds from the Titan.

OceanGate's past expeditions to the Titanic wreck site have included archeologists and marine biologists. The company also brings people who pay to come along.

OceanGate's website described the "mission support fee" for the 2023 expedition as $250,000 a person. Passengers, who are called "missions specialists" by the company, take turns operating sonar equipment and performing other tasks in the submersible.

As of Tuesday, there was less than 40 hours of breathable air left in the sub while rescue crews scramble to retrieve it.

Associated Press writers Danica Kirka, Jill Lawless and Sylvia Hui in London, Rob Gillies in Toronto, Olga R. Rodriguez in San Francisco, Jon Gambrell in Dubai, United Arab Emirates, and Munir Ahmed in Islamabad contributed to this report.