(NewsNation) — The oxygen supply on a submersible watercraft lost at sea is believed to have run out Thursday morning, with still no sign of the vessel that was diving to explore the wreckage of the Titanic.

The U.S. Coast Guard and Canadian authorities have been searching for the sub since Monday, a day after it stopped communicating with its mothership on the surface in the Atlantic Ocean.

The primary search area is vast — two times the size of Connecticut — 900 miles east off the coast of Cape Cod, 400 miles south of Newfoundland, Canada, where the crew launched its research expedition last Friday.

As of Wednesday, Coast Guard officials said it was a search-and-rescue mission. They have not changed that status as of publication time Thursday.

Here’s what we know about the search right now:

What equipment has been deployed?

There is a coalition of military and private sector ships scouring the waters, including U.S. Coast Guard and Canadian ships, remote-operated vehicles and commercial research vessels. In the sky are U.S. and Canadian military planes. Sonar buoys are also in the water.

The U.S. Navy is sending its “flyaway deep ocean salvage system” in case the submersible is found and needs a lift out of the water. The Coast Guard said the equipment was scheduled to arrive in St. John’s, Canada, on Wednesday night.

Three vessels arrived Wednesday: Canadian Coast Guard boat John Cabot, which has sonar capabilities, as well as commercial vessels Skandi Vinland and the Atlantic Merlin.

Even more were en route as of Wednesday night, the Coast Guard said in a news release. They include:

• Canadian CGS Ann Harvey 
• Canadian CGS Terry Fox 
• Motor Vessel Horizon Arctic (remote-operated vehicle) 
• French Research Vessel L’Atalante (remote-operated vehicle) 
• His Majesty’s Canadian Ship Glace Bay (mobile decompression chamber and medical personnel) 
• Air National Guard C-130  
• Remote-operated vehicle from Magellan, the company that mapped the Titanic wreck

The Polar Prince, the research vessel from which the Titan launched, has remained in the water for the duration of the search. The Deep Energy research vessel is also searching.

‘Underwater noises’ detected

Late Tuesday night, the U.S. Coast Guard said “underwater noises” were detected by a Canadian P-3 aircraft. Remotely operated vehicles were relocated to investigate, but nothing was found.

On Wednesday, authorities said it was unclear what those noises could have been. The Navy is analyzing the sounds to determine if they were man-made or biological.

“I want to reiterate this is a very complex search and the unified team is working around the clock to bring all available expertise and assets to bear as quickly as possible in an effort to solve this very complex problem,” Coast Guard Capt. James Frederick said Wednesday.

Retired U.S. Navy Capt. Brad McDonald said there are a lot of sounds in the ocean, but a metallic sound made by a possible banging by another piece of metal will travel miles and miles underwater.

“In the old cat and mouse games that submarines play, being the quietest submarine is usually the way to win the game,” McDonald explained. “And you can have the quietest submarine in the world. But if a sailor drops a wrench in a bilge, that noise will travel for miles.”

McDonald said when a metallic noise is picked up on sonar on a Navy submarine, it’s very clear to the crew it’s most likely another submarine.

Who is on board?

The Coast Guard said there were one pilot and four “mission specialists” aboard. “Mission specialists” are people who pay to come along on OceanGate’s expeditions. They take turns operating sonar equipment and performing other tasks in the five-person submersible.

Among the passengers of the submersible is Hamish Harding, a 58-year-old British businessman. Harding lives in Dubai in the United Arab Emirates, according to Action Aviation, a company for which Harding serves as chairman.

The CEO and founder of OceanGate Expeditions, Stockton Rush, is also among the passengers, along with Pakistani businessman Shahzada Dawood and his son Suleman. The fifth person is believed to be Paul-Henri Nargeolet, the submersible’s pilot.

Jannicke Mikkelsen, a friend and colleague of Harding, said this week Harding is an “explorer at heart.”

“This was yet another goal on his checklist of what he wanted to explore down here on Earth, and also in space,” Mikkelsen said Monday on “CUOMO.” “Titanic was one of the big high points that he wanted to achieve in his life.”

Greg Stone, a longtime ocean scientist and a friend of Rush, called him “a real pioneer” in the innovation of submersibles.

“Stockton was a risk-taker. He was smart. He was, he had a vision, he wanted to push things forward,” Stone said Tuesday.

Father-and-son Shahzada and Suleman Dawood are members of one of Pakistan’s most prominent families. Their family said in a statement that they were both aboard the vessel.

“We are very grateful for the concern being shown by our colleagues and friends and would like to request everyone to pray for their safety while granting the family privacy at this time,” the statement said. “The family is well looked after and are praying to Allah for the safe return of their family members.”

Nargeolet is a former French navy officer who is considered a Titanic expert after making multiple trips to the wreckage over several decades.

What is the Titan submersible?

Built by OceanGate Expeditions, the Titan is intended for deep-sea explorations. The company offers trips to see the Titanic wreckage, according to its website.

An initial group of tourists in 2021 paid $100,000 to $150,000 apiece to go on the trip. OceanGate’s website described the “mission support fee” for the 2023 expedition as $250,000 a person.

The Titan can hold five passengers and dive to a depth of just over 13,000 feet. Made of carbon fiber and titanium, it weighs about 23,000 pounds on land, but is neutrally buoyant once it reaches the seafloor, OceanGate says.

The vessel is equipped with sophisticated technology including a “proprietary real-time hull health monitoring system (RTM)” that analyzes the pressure on the vessel and the integrity of its structure.

There is only one window on the vessel, where quarters are cramped. Passengers have no room to stand, and there is one toilet.

If there was a power failure, the submersible would be incredibly dark — and cold — at the bottom of the ocean.

Safety concerns about vessel

Details continue to come to light about safety warnings made directly to OceanGate leading up to the Titan’s disappearance. There were concerns raised about dangers when the vessel reached extreme depths dating back to 2018.

OceanGate was repeatedly warned that there might be catastrophic safety problems posed by the way the Titan was developed, court documents show.

David Lochridge, OceanGate’s director of marine operations, wrote an engineering report in 2018 that said the craft under development needed more testing and that passengers might be endangered when it reached “extreme depths,” according to a lawsuit filed that year in U.S. District Court in Seattle.

OceanGate sued Lochridge that year, accusing him of breaching a non-disclosure agreement, and he filed a counterclaim alleging that he was wrongfully fired for raising questions about testing and safety. The case settled on undisclosed terms several months after it was filed.

In an interview with CBS News last year, Rush defended the safety of his submersible but said nothing is without risk.

“What I worry about most are things that will stop me from being able to get to the surface — overhangs, fish nets, entanglement hazard,” he said, adding that a good pilot can avoid such perils.

NewsNation writers Devan Markham and Urja Sinha, correspondent Dray Clark and The Associated Press contributed to this report.