As of Tuesday afternoon, international rescue crews are still searching the North Atlantic for a submersible vessel that was taking five people to view the wreckage from the Titanic.
Authorities estimate there's only enough oxygen in the sub for those onboard to survive less than two days.
Here's what we know.
When and where did the vessel go missing?
The 21-foot vessel, which is named Titan, lost communication with its control center on Sunday morning, roughly 1 hour and 45 minutes into its scheduled dive, the U.S. Coast Guard wrote on Twitter.
Titan had been deployed by a Canadian expedition ship, the Polar Prince, about 435 miles (380 nautical miles) south of St. John's in Newfoundland, not far from the site of the iconic shipwreck.
Why was the sub diving?
The missing vessel is owned by OceanGate, a company based in Washington state that offers underwater voyages to explore the remains of the Titanic from the seafloor.
OceanGate is a major chronicler of the ship's decay and shared the first-ever full-size digital scan of the wreck site in May.
OceanGate is also a pioneer in the deep sea tourism economy. For $250,000 a person, the company takes adventurers on a deep sea tour lasting eight days and stretching hundreds of miles.
From St. John's in Newfoundland, Canada, explorers travel 380 miles offshore and 2.4 miles below the surface.
If successful, they can catch a glimpse of what's left of the 1912 iceberg-crash disaster, which took the lives of all but 700 of the Titanic's 2,200 passengers and crew. Today, the ship is slowly succumbing to a metal-eating bacteria, which may cause it to fully disintegrate in a matter of decades.
Mike Reiss, who joined OceanGate to glimpse the deteriorating wreck in 2022, said the trip is less tourism than it is true exploration — and the people who dare to try it are made well aware of the risks.
"You sign a massive waiver that lists one way after another that you could die on the trip," he told the BBC in an interview Tuesday. "They mention death three times on page one. So it's never far from your mind. As I was getting on to the sub, that was my thought: That this could be the end."
Who was on board?
The Titanic-touring vessel contained one pilot and four paid passengers called "mission specialists," according to the U.S. Coast Guard. "Mission specialists" take turns operating sonar equipment and performing the tasks necessary to complete a dive.
Among those paid passengers was British businessman Hamish Harding, according to a statement from Action Aviation, a company where Harding works as chairman.
Harding holds three Guinness World Records, including the longest duration (4 hours, 15 minutes) at a full ocean depth (2.88 miles) by a crewed vessel. He has also trekked to the South Pole, circumnavigated the Earthin less than 48 hours and visited space in Blue Origin's New Shepard rocket.
Shahzada Dawood and his son Suleman, two members of a prominent Pakistani family known for investing, were also on board the vessel, according to a statement shared with outlets such as The Associated Press.
A fourth person on board is Paul-Henri Nargeolet, a French expert on the Titanic, his agent has confirmed to several outlets, including The New York Times. Nargeolet serves as director for RMS Titanic Inc., the U.S. company that owns the salvage rights to the Titanic site. The Times reports that Nargeolet has completed over 35 dives to the wreckage, including a previous Titan expedition.
Titan's pilot has yet to be identified.
Why did the vessel go missing?
It's still unclear why the sub lost communication with its control crew on the expedition ship.
Ahead of its launch, OceanGate said it would rely on the satellite-based internet company Starlink for the communications necessary for carrying out the expedition. A text-message-based system relying on underwater acoustic positioning normally allows the Titan to communicate with the control ship, according to an Australian researcher writing for The Conversation.
OceanGate says its vessels are "equipped with some basic emergency medical supplies and 96 hours of life support," according to the company's website.
And for good reason: This is not the first time an OceanGate submersible has gotten lost, according to David Pogue, a correspondent for CBS Sunday Morning.
Pogue, who traveled on an OceanGate expedition to see the Titanic last summer, recalled that the control room was unable to help the submersible locate the wrecked liner for roughly three hours due to technical difficulties.
"The difference this year is that it seems like they lost contact with the ship," Pogue told NPR. "They can't even reach the sub and that's really scary."
He added that factors like bad weather and mechanical issues mean the submersible vessels rarely make it to the Titanic, despite the expensive price tag. This season has seen zero successful dives, Pogue said.
What's it like inside the Titan?
Videos from Pogue's initial CBS Sunday Morning report on OceanGate show him reading from the "mission specialist" waiver, which points out that Titan has not been approved or certified "by any regulatory body."
"I couldn't help noticing how many pieces of this sub seem improvised," Pogue adds.
A single plastic bottle and some Ziploc bags stand in for a toilet. An Xbox game controller and an elevator-esque up/down button serve as the vessel's primary controls. The interior lighting is from Camping World, notes OceanGate founder Stockton Rush.
In whole, the space inside is about the size of a minivan, not tall enough for someone to fully stand.
In an interview with NPR's All Things Considered, Pogue said there are seven different ballast mechanisms that can help the Titan rise from great depths.
"Some of these work even if the power is out or even if everyone on board is passed out," Pogue said.
The fact that rescue crews haven't spotted the vessel on the ocean's surface might mean that the Titan is snagged or its 5-foot-thick carbon fiber hull was penetrated, Pogue said.
Either situation could be catastrophic for the people on board.
What's the latest on the search efforts?
As of Tuesday at 1 p.m. ET, the Titan had about 40 hours of oxygen left, said Capt. Jamie Frederick, a response coordinator for the U.S. Coast Guard overseeing the search.
A unified command including the U.S. Coast Guard, the U.S. Navy, the Canadian Coast Guard and OceanGate are working together to steer the search efforts, but so far, has come up unsuccessful, Frederick said during a press conference.
"We wouldn't be searching and putting all effort out there" if the sub wasn't recoverable, Frederick said, adding that the crews contain the "nation's best experts."
Several Canadian Coast Guard vessels were en route to the scene, as were several additional private vessels.
The teams in place are continuing to use aircraft to scan the ocean, an effort that may get easier on Tuesday as Monday's heavy fog was lifting, said a Coast Guard spokesperson.
Sonar devices are also being employed to detect possible underwater sounds coming from the submersible. Crews have covered 7,600 square miles — an area bigger than the state of Connecticut.
The teams also expanded their underwater search capability on Tuesday by adding a remotely operated vehicle in order to reach lower depths. That search is ongoing.
But even if the crews can locate the vessel at a low depth, hauling it up to the surface is another task. The Titan could be at a depth of over 13,000 feet and a distance of over 900 miles offshore. Frederick said the search and rescue crews did not yet have salvage equipment in place.
David Marquet, a retired U.S. Navy submarine captain, told NPR's Morning Edition that the odds of survival are "about 1%."
NPR's Juliana Kim contributed reporting.
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