US and Canadian search teams are racing against time to find a tourist submersible that went missing during a dive to the Titanic’s wreck on Sunday.
Five people were onboard when contact with the small sub was lost about an hour and 45 minutes into its dive.
The rescue operation is continuing overnight in the mid-Atlantic but there has been no sign so far of the vessel.
Government agencies, both countries’ navies and commercial deep-sea firms are all helping the rescue operation.
As of Monday afternoon, it was thought the crew members had roughly four days-worth of oxygen left. Among them is the British billionaire businessman and explorer Hamish Harding.
Pakistani businessman Shahzada Dawood and his son, Suleman, were also on board, their family said in a statement.
French explorer Paul-Henry Nargeolet is thought to be on board, according to a Facebook post by Mr Harding before the dive started
Stockton Rush, chief executive of OceanGate – the firm behind the dive – is also being widely reported to be on the vessel.
“Right now, our focus is on getting as much capability into the area as we can,” Rear Adm John Mauger of the US Coast Guard told a press conference.
Military planes, a submarine and sonar buoys have so far been used in the search for the vessel.
Titanic’s wreck lies some 435 miles (700km) south of St John’s, Newfoundland, though the rescue mission is being run from Boston, Massachusetts.
The US Coast Guard said a research ship called the Polar Prince had conducted a surface search for the sub on Monday evening. It is used to transport submersibles to the wreckage site and was the support ship on Sunday’s tourist expedition.
The missing craft is believed to be tour firm OceanGate’s Titan submersible, which CBS journalist David Pogue travelled aboard last year to reach the wreckage of the Titanic.
He has told the Ocean press that when the support ship is directly above the sub, short text messages are able to be sent between the two.
Otherwise, communication via GPS or radio systems is not available as neither work underwater.
Pogue says it is also not possible for those aboard the sub to escape by themselves because they are sealed inside by bolts applied from the outside.
A submersible vessel is different to a submarine. According to the National Oceanic and Atmospheric Administration, a submarine can launch itself into the ocean from a port independently, while a submersible has very limited power reserves so needs a mother ship that can launch it and recover it.
Rear Adm Mauger noted the area in which the search was taking place was “remote”, making operations difficult.
Added to this is the fact that visibility is quickly lost below the surface of the water as light cannot penetrate far.
The OceanGate website lists three submersibles it owns, and only the Titan is capable of diving deep enough to reach the Titanic wreckage.
The vessel weighs 23,000 lbs (10,432 kg) and, according to the website, can reach depths of up to 13,100 ft.
Tickets cost $250,000 (£195,000) for an eight-day trip including dives to the wreck at a depth of 3,800m (12,500ft).