A submersible carrying five people to see the remains of the Titanic at the bottom of the North Atlantic Ocean is still missing despite a massive search operation by United States and Canadian authorities.
The Titan submersible had been on its way to the famous wreckage, off the coast of St John's, Newfoundland, in Canada, when it lost contact with its support ship, leaving the people on board with only enough oxygen for 3 to 4 days.
Here's what we know so far.
How did they go missing?
The submersible was part of an eight-day journey conducted by OceanGate Expeditions. The trip is based out of Newfoundland, with participants first traveling 400 nautical miles to the wreck site, which is about 900 miles (1,450 kilometers) off the coast of Cape Cod, Massachusetts.
The submersible began its two-hour descent to the wreck on Sunday morning. It lost contact with the Polar Prince, the support ship that transported the vessel to the site, 1 hour and 45 minutes into its descent, officials said.
Search operations began later that day.
It's still not clear what happened to the submersible, why it lost contact, and how close to the Titanic it was when it went missing.
Who's on board?
The five people on board include one pilot and four "mission specialists," said Rear Adm. John Mauger, commander of the US Coast Guard's First District.
A British businessman based in the United Arab Emirates, Hamish Harding, is one of the people on board, according to a social media post by the company he owns, Action Aviation.
"The sub had a successful launch and Hamish is currently diving," the company said in an Instagram post on Sunday.
Harding was one of the first people to travel the Challenger Deep in the Pacific Ocean -- the deepest known point on Earth. On Saturday he wrote of the Titanic mission: "I am proud to finally announce that I joined OceanGate Expeditions for their RMS TITANIC Mission as a mission specialist on the sub going down to the Titanic."
The other passengers on board have not yet been publicly identified, with authorities still in the process of notifying their families.
But Harding had posted on social media Saturday saying that diver Paul-Henry (P.H.) Nargeolet was scheduled to be on Sunday's dive with him. Nargeolet has led several expeditions to the Titanic and supervised the recovery of many artifacts from the wreck, according to the E/M Group, where Nargeolet was director of underwater research.
CNN has attempted to reach out independently to Nargeolet with no success.
The OceanGate website says its expeditions, which cost upwards of $250,000, typically involve one pilot, a "content expert" and three paying passengers.
How much time do they have?
Coast Guard officials estimated on Monday afternoon the submersible had "somewhere between 70 to the full 96 hours" of oxygen -- potentially giving rescuers until Friday to locate and retrieve the vessel.
But the depth of the area where they went missing could pose a challenge. The deepest ever underwater rescue was that of Roger Chapman and Roger Mallinson, who were rescued from the Pisces III submersible at depths of 1,575 feet in 1973. They were trapped for 76 hours before finally being hauled to the surface.
The Titanic wreckage is much deeper, sitting nearly 13,000 feet below sea level.
Other factors complicating the search include its remote location, local weather conditions, and unknowns like the state of the submersible and whether it has working equipment like acoustic pingers that can be detected by search teams.
According to CNN meteorologists, the area is seeing choppy seas up to six feet high, and foggy conditions. While not unusual for the region, this could cause delays for search parties due to difficulty using aerial equipment in low clouds.
What is the submersible?
According to OceanGate, Titan is a 23,000-pound submersible made of carbon fiber and titanium.
As a safety feature, the sub uses a "proprietary real-time hull health monitoring (RTM) system" that analyzes the pressure on the vessel and the integrity of the structure, the company states. Any issues detected would trigger an "early warning" to the pilot, to leave "enough time to ... safely return to the surface."
Unlike a submarine, a submersible has limited power reserves so it needs a mother ship that can launch and recover it, according to the National Oceanic and Atmospheric Administration.
When CBS correspondent David Pogue took a trip on the Titan down to the Titanic wreck last year, on the invitation of OceanGate, he said the hatch was sealed from the outside with 17 bolts -- there was no other way out.
With no GPS underwater, the submersible is only guided by text messages from the surface ship. On Pogue's trip, communications broke down during a dive and the submersible was lost for over two hours, he said.
Thomas Shugart, a former US Navy submarine captain and analyst at the Center for a New American Security, told CNN it didn't bode well that search parties still haven't heard from the missing submersible.
"If they'd had a relatively minor issue that forced them to surface unexpectedly, one would think that a locator beacon would have been detected by now. If instead they are stuck on the bottom for some reason, I have yet to hear of a rescue capability that could get them back in time," he said.
What search operations are underway? How might it be rescued?
The US Coast Guard tweeted on Monday that the search will continue during the night. It began scanning the ocean surface on Sunday when the submersible was first reported missing.
Boats, aircraft and radar equipment were deployed to scan above water in case the submersible had surfaced. Sonar buoys and sonar equipment in commercial vessels onsite were also being used to detect sounds below water.
The US Coast Guard is coordinating the search with the US Navy, the Canadian Coast Guard and the Canadian military.
The US Air Force's 106th Rescue Wing has flown over the area, as have two C-130 flights. The Canadian Coast Guard subsurface and surface search will continue Tuesday morning.
OceanGate said it has received assistance from government agencies and deep sea companies. The Polar Prince, the ship that was supporting the submersible, is also assisting with the search, a co-owner said.
But finding the submersible is just the first step; rescuing it could be a whole other challenge.
Depending where and at what depth the submersible is found, there could be limited options for rescue vessels.
For instance, the US Navy's nuclear-powered submarines usually operate at 800 feet or less -- meaning they can't dive down to the ocean floor, where water pressure on the submarine hull could make it implode.
The Navy does have specialized rescue submersibles, but even those can only make rescues at depths up to 2,000 feet, according to the Navy's Underwater Rescue Command.
During the 1973 rescue, authorities used other submersibles and a remotely operated, Navy-developed recovery vessel to attach lines to the Pisces III -- which were then used to pull it back to the surface.
It's not clear whether these methods could work for the Titan, given the uncertainty around its location.
