- A full dive to the wreck, including the descent and ascent, reportedly takes eight hours.
- Each expedition lasts eight days, according to OceanGate, and each dive is meant to include a scientific objective, including studying the wreck's decay.
A massive search and rescue effort is under way in the North Atlantic after a submersible exploring the wreck of the Titanic went missing on Sunday.
The research vessel Polar Prince lost contact with the crew of the Titan sub an hour and 45 minutes into its dive. The US Coast Guard estimated the sub had between 70 and 96 hours of emergency oxygen, as of 17:00 EST (22:00 BST) on Monday.
Tour firm OceanGate said it was exploring all options to get the crew back safely, and government agencies have joined the rescue operation.
Here is what we know so far.
What is the latest on the rescue effort?
On Tuesday morning, US Coast Guard Rear Adm John Mauger said that the search area had been expanded to deeper waters after a night of searching that ultimately proved unsuccessful.
David Mearns, a marine scientist and expedition leader who has been following the operation, also told the BBC that a commercial pipe-laying ship had arrived in the area to help.
"We're just hoping that it has the capabilities to reach those kind of depths - 3,800 metres - to search for the submersible and have the ability to recover it," he said. "There is some hope that that could happen."
The search has now stretched into its second day. The Polar Prince first arrived near the Titanic wreck on Sunday morning and the Titan was scheduled to start its drive early that day at 04:00 local time.
The Boston Coast Guard, which is leading the search operation, said on Twitter that "the crew of the Polar Prince lost contact with them approximately one hour and 45 minutes into the vessel's dive".
The Titan submersible was thought to be approximately 900 miles (1450km) off the coast of Cape Cod at the time.
The US Coast Guard's Rear Admiral John Mauger said on Monday that it is a challenge to conduct a search in such a remote area.
The search has two aspects, he said. These are a surface search in case the Titan has returned to the ocean's surface but somehow lost communications, and an underwater sonar search.
The Coast Guard has sent two C-130 Hercules aircraft to search for the submersible on the surface of the water, and has been joined by a Canadian C-130, and a P8 aircraft equipped with underwater sonar capability. Sonar buoys are also being deployed in the area.
Mr Mauger said that additional expertise would be needed to rescue the vessel if it was found underwater, and that the Coast Guard has reached out to the US Navy and private sector for help.
Canada's defence department said that, along with the aircraft, the Canadian coast guard vessel Kopit Hopson was assisting in the search.
Mike Mulroy, a former US undersecretary of defence, told the BBC that specialised US Navy remote submersibles would likely be the only vessels capable of tethering with the submarine and dragging it to the surface.
Additionally, Horizon Maritime, which co-owns the Polar Prince, confirmed to the BBC that the vessel is helping and a second vessel, the Horizon Arctic, has been sent to the site.
Who was on board?
Among the five people on board the Titan, four people so far have been confirmed - Hamish Harding, a 58-year-old British businessman and explorer, British businessman Shahzada Dawood and his son, Suleman Dawood, and French explorer Paul-Henry Nargeolet.
Mr Harding announced he was joining the team earlier this month, and said that the crew on the vessel includes "a couple of legendary explorers, some of which have done over 30 dives to the RMS Titanic since the 1980s".
Mr Harding is the chairman of Action Aviation, an international company which deals with sales and operations in the business aviation industry, based in Dubai, United Arab Emirates.
French explorer Paul-Henry Nargeolet was also on board, a family spokesman told the BBC. The explorer, nicknamed "Mr Titanic", is a former French navy diver and has reportedly spent more time at the wreckage of the Titanic than any other explorer.
And Stockton Rush, chief executive of OceanGate - the firm behind the dive - is also being widely reported to be on the vessel.
What was the sub doing and where is the Titanic?
OceanGate Expeditions charges guests $250,000 (£195,270) for a place on its eight-day expedition to see the famous wreck, which sits 3,800m (12,500ft) beneath the surface at the bottom of the Atlantic.
The site is about 600km (370 miles) off the coast of Newfoundland, and lies in two parts, with the bow and the stern separated by about 800m (2,600ft). A huge debris field surrounds the broken vessel.
A full dive to the wreck, including the descent and ascent, reportedly takes eight hours.
Each expedition lasts eight days, according to OceanGate, and each dive is meant to include a scientific objective, including studying the wreck's decay.
The inaugural dive took place in 2021, according to the company's website.
What might have happened to it?
Submarine expert Prof Alistair Greig from University College London has worked through a number of scenarios for where the missing submersible might be, he told BBC Science Correspondent Pallab Ghosh.
One is that it released a "drop weight" after an emergency, in order to bring it to the surface.
"If there was a power failure and/or communication failure, this might have happened, and the submersible would then be bobbing about on the surface waiting to be found," he said.
Another scenario, he says, is that the hull was compromised resulting in a leak. "Then the prognosis is not good."
If it has gone down to the seabed and can't get back up under its own power, the options - according to Prof Greig - are very limited.
"While the submersible might still be intact, if it is deeper than more than 200m (656ft) there are very few vessels that can get that deep, and certainly not divers. The vehicles designed for navy submarine rescue certainly can't get down to anywhere near the depth of the Titanic."
What do we know about the Titan?
The Titan is a five-person submersible built to descend to depths of 4,000 metres (2.5 miles) and travels at three knots - that's about 3.5 miles per hour.
Aside from taking divers to the wreck of the Titanic, it's used for site survey and inspection, research and data collection, film and media production, and deep sea testing of hardware and software.
According to the company, the Titan is "outfitted with state-of-the-art lighting and sonar navigation systems plus internally and externally mounted 4K video and photographic equipment".
According to OceanGate's website, the five-person sub has a system for monitoring the vessel's hull in real time.
It has sensors to analyse the effects of changing pressure on the sub as it dives, in order to assess the integrity of the structure.
"This onboard health analysis monitoring system provides early warning detection for the pilot with enough time to arrest the descent and safely return to surface," the company says.
The BBC's US partner CBS sent one of its reporters on a voyage with the same company last year to see the wreck of the Titanic.
David Pogue, who went on board, reported that he read a waiver that described the submersible as an "experimental" vessel, "that has not been approved or certified by any regulatory body, and could result in physical injury, disability, emotional trauma or death".
Mr Rush then gave him a tour of the submersible, where he revealed the vessel only has one button and is run using a video game controller.