Rescue teams are continuing their hunt Thursday for a missing submersible that had planned to visit the wrecked Titanic site.
Remote-operated vehicles are still searching for the sub, called Titan, near the site where a surveillance aircraft first detected "underwater noises" late Tuesday.
The 21-foot Titan is carrying five people who hoped to see the famous wrecked ocean liner on Sunday. If still intact, the vessel's oxygen supply may run out early Thursday morning, according to estimates from those involved in the search.
Here's a guide to what we know.
Have officials discovered the source of the "underwater noises"?
The U.S. Coast Guard said that maritime surveillance planes operated by Canada detected underwater sounds late Tuesday, then again on Wednesday.
Various underwater search efforts had been moved to the location of the noise to discover its source, but that so far, underwater drones operated remotely had "yielded negative results," the Coast Guard said.
Underwater acoustic experts from the Navy are analyzing the sounds, which one expert described as "banging noises," but the Coast Guard and other experts with knowledge of the search are warning that the sounds may not necessarily be proof of life.
"You have to remember that it's the wreck site of the Titanic, so there is a lot of metal and different objects in the water around the site," Rear Adm. John Mauger said in an interview with CBS News on Wednesday morning.
Where and how is the search unfolding today?
The U.S. Coast Guard says that the data from the Canadian aircraft, known as a P-3 Orion, is nevertheless serving as a focus point for its unified search efforts.
The remoteness of the location and the size of the search area — extending 10,000 miles on the surface and 2.4 miles down to the ocean floor — has complicated efforts to locate the vessel and its passengers.
At least one remote-operated vehicle, deployed by the Canadian vessel Horizon Arctic, reached the sea floor on Thursday, and another, the French vessel L'Atalante, is expected to deploy on Thursday, the U.S. Coast Guard said.
Multiple maritime surveillance planes from U.S. and Canadian armed forces have flown over the site and dropped sonar buoys in the area.
Also en route is the U.S. Navy's Flyaway Deep Ocean Salvage System (FADOSS), a motion-compensated lift system which can lift heavy undersea objects.
The design of the Titan means that only those outside the vessel can unseal it, so regardless of whether it rises to the surface or not, the passengers will require outside help to escape.
The Canadian ship Glace Bay, which contains a mobile decompression chamber and is staffed with medical personnel, is also expected to arrive Thursday morning.
When will the sub's oxygen supply run out?
At the beginning of the search on Sunday, officials estimated the submersible, if still fully functional, contained about 96 hours of reserve oxygen. At a press conference on Wednesday, the Coast Guard estimated that supply to be down to about 20 hours.
That means the the oxygen on board the Titan could run out early Thursday morning.
But U.S. Coast Guard Capt. Jamie Frederick said that's only "one factor" in what is still "a search-and-rescue operation, 100 percent."
"We need to have hope," he said in a Wednesday media briefing. He added that he wasn't ready to speculate about when the search efforts might end.
When and where did the sub go missing?
Titan lost contact with its support ship — a Canadian research vessel called Polar Prince — an hour and 45 minutes after it first entered the water on Sunday around 8 a.m. ET.
At that point, it was already more than halfway down to the Titanic's wreck on the Atlantic's ocean bed, roughly 900 miles east of Cape Cod in Massachusetts.
The Coast Guard said it was first notified of the missing vessel at 5:40 p.m. ET, nearly three hours after the Titan was expected to resurface at 3 p.m. ET.
Who was on board?
The people on board Titan include pilot Stockton Rush, the head of OceanGate, the company that developed the submersible; Paul-Henri Nargeolet, a French underwater wreck expert who has written about the Titanic and visited the wreck dozens of times; a British entrepreneur Hamish Harding; and father-son Pakistani nationals Shahzada and Suleman Dawood.
A former passenger of the Titan described the vessel as like being in a "minivan without seats" and says its interior design relies on "off-the-shelf parts," including a video game controller for steering.
What was the sub's mission?
The missing vessel is owned by OceanGate, a company based in Washington that's become a major chronicler of the Titanic's decay.
In May, OceanGate shared the first-ever full-size digital scan of the wreck site, which is slowly succumbing to a metal-eating bacteria and at risk of disintegrating in a matter of decades.
For $250,000 a person, the company promises tourists an underwater voyage to explore the remains of the Titanic from the seafloor. From St. John's in Newfoundland, Canada, explorers travel 380 miles offshore and 2.4 miles below the surface. A full trip can take eight days and include multiple dives.
If successful, the dives offer a glimpse of what's left of the 1912 crash into an iceberg, which took the lives of all but 700 of the Titanic's 2,200 passengers and crew.
Did anyone warn OceanGate that the Titan wasn't safe?
Years before the Titan went missing, OceanGate faced several complaints and warnings about the safety of its submersible vessels.
Records from a 2018 lawsuit show that the company's former director of marine operations, David Lochridge, flagged potential safety issues with the Titan as it was under development in 2015.
Lochridge was particularly concerned about the company's lack of testing on the Titan's 5-inch-thick carbon fiber hull, which employed an experimental design developed in collaboration with NASA. He also said that the Titan's port window was only designed to withstand depths of about 4,200 feet — far shallower than the 13,000-foot depth of the Titanic.
OceanGate responded in legal filings by saying it relied on acoustic testing "better suited" to detect safety issues. The company fired and sued Lochridge, accusing him of breaching his contract.
Separately, but in the same year the lawsuit was settled, the chairman of the Marine Technology Society's Submarine Group wrote a letter to OceanGate saying 38 industry experts had "unanimous concern" about the Titan's lack of adherence to industry standards.
"We have submarines all over the world diving at 12,000 to 20,000 feet every day of the year, for research. We know very well how to design these machines and operate them safely," the chairman, Will Kohnen, told NPR's Morning Edition on Wednesday.
How did OceanGate respond to warnings about Titan's safety?
OceanGate has seen at least two documented safety incidents with the Titan after these warnings.
During a 2022 expedition, OceanGate reported that its sub had experienced a battery issue during a dive and had to be manually reattached to its lifting platform, court filings show.
In the same year, the vessel lost contact with its surface crew for nearly five hours during a dive, according to CBS correspondent David Pogue, who was observing the mission for a journalistic report on the company.
Pogue reported that a waiver for passengers of the Titan clearly states the vessel has not been approved or certified by any regulatory body.
OceanGate's founder, who is reportedly on board the missing Titan, said in a 2019 interview that the commercial submarine industry's regulations stood in the way of progress.
"It's obscenely safe because they have all these regulations," Rush told The Smithsonian Magazine. "But it also hasn't innovated or grown."
NPR's Willem Marx, Ayana Archie and Juliana Kim contributed reporting.
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