By Andrew Jeong and Kyle Melnick
A submersible vessel carrying five people in the North Atlantic went missing on Sunday, triggering a search and rescue effort.
The Titan underwater vehicle, owned by OceanGate Expeditions, was exploring the wreckage of the Titanic when it lost contact with a research vessel on the surface.
The missing people probably have enough oxygen to survive for three to four days, Rear Adm. John Mauger, who is leading the Coast Guard's search operation, told reporters.
But the remote location and depth of the Titanic site, combined with debris in the area, could hamper rescue efforts.
Humans can't survive without oxygen at the Titanic's depth, said Andrew Norris, a retired Coast Guard captain, adding that he doesn't believe a rescue vessel could reach that depth, either.
When did the submersible vehicle go missing?
The submersible went on a dive on Sunday morning and lost contact with the Canadian research vessel Polar Prince about 900 miles east of Cape Cod.
OceanGate alerted the Coast Guard on Sunday afternoon after contact was lost about one hour and 45 minutes into the dive.
A US Coast Guard C-130 aircraft and a Canadian P-8 aircraft able to search with underwater sonar equipment were looking for the submersible. A US Air Force C-130 and three ships including the Polar Prince had joined the search on Monday, according to flight and marine traffic trackers.
What is the Titan submersible?
The missing submersible, known as the Titan, is designed to take five people - one pilot and four crew members - to depths of about 13,100 feet (about 3 900m).
The vessel has 96 hours of life support for five people. It's about the size of a minivan, CBS correspondent David Pogue said after joining an expedition last year, and is steered using a video game controller.
It operates with a launch-and-recovery platform and, in the current expedition, was paired with the Canadian research vessel Polar Prince on the surface. (A submersible is different from a submarine in that it is supported by a surface vessel, platform, shore team or submarine.)
Space is tight on the vessel, and equipment is minimal. So are the facilities: Though there is a small toilet separated by a privacy curtain, OceanGate has advised passengers to "restrict your diet before and during the dive to reduce the likelihood that you will need to use the facilities."
What is OceanGate Expeditions, the company operating the missing Titan sub?
OceanGate Expeditions, led by CEO Stockton Rush, is a private research and tourism company that has organised more than a dozen underwater expeditions since 2010, including to the SS Andrea Doria south of Nantucket.
Rush is a co-founder and a board member of OceanGate Foundation, a non-profit that aims to use emerging technology to further research in marine science, history and archaeology.
OceanGate also explored the Titanic wreck in 2021 and last year, documenting its rate of decay.
The company has said it planned to return annually. Because of the large scale of the wreck and the debris field, missions aimed at collecting images, videos, and laser and sonar data over several years would be required, it said.
Rush told CBS News last year that OceanGate's eight-day expeditions cost $250,000 (about R4.5m) for every person who dives down to see the wreckage.
Rush said some of the Titanic enthusiasts - or "Titaniacs" - who travel on the submersible mortgage their homes, while others are so rich they don't think twice about the cost. The company has had one client who had won the lottery.
How deep is the Titanic wreckage, and where is it located?
The wreckage is in a remote area about 370 miles south of Newfoundland, Canada, at a depth of 12 500 feet, or about 2.4 miles. More than 1 500 people are estimated to have been killed when the Titanic sunk in 1912.
Humans can't survive without oxygen at that depth. Norris, the retired Coast Guard captain, said he doesn't believe a rescue vessel could reach that far below the surface.
The search will take place on the surface - in case the vessel was able to come up but lost communications - as well as underwater, with the use of sonar buoys and ship sonar, Mauger said.
As rescuers look for the submersible, they will have to comb through "a debris field two-plus miles below the surface" among other areas, according to Norris. "And then get rescue vessels or vehicles down to the scene, if there is even one that's available or even one that exists."
Who is on the missing submersible?
OceanGate and government officials have not released the names of the passengers.
But at least one passenger posted on social media before embarking: "I am proud to finally announce that I joined @oceangateexped for their RMS TITANIC Mission as a mission specialist on the sub going down to the Titanic," Hamish Harding recently wrote on his Instagram page.
Harding, a British businessman and seasoned adventurer, has flown to space on a flight organised by Blue Origin, a space venture founded by Jeff Bezos.
The family of Pakistani businessman Shahzada Dawood and his son, Suleman, also released a statement saying the pair are on the expedition: "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."
What is submersible tourism?
Since the 1980s, tourists have explored oceans by riding in submersibles. Companies and resorts near reefs, such as in Hawaii and the Caribbean, allow tourists to rent submersibles to experience the sights of scuba diving without swimming or getting wet.
Submersibles differ in size, fitting one person or as many as 60. For tourists, the vehicles often dive about 150 feet but can plunge deeper.
Some submersibles are operated by a pilot, while others are programmed to travel a designed route.
OceanGate has held scientific and tourism expeditions in submersibles in Washington, California, the Gulf of Mexico, Massachusetts, Florida and Canada.
The Washington Post's Tamia Fowlkes and Maham Javaid contributed to this report.
Read the latest issue of TRAVEL digital magazine here.