The US Coast Guard was racing against time on Tuesday and facing a host of extreme logistical challenges, including crushing pressure deep below the ocean, to find a deep-diving submersible and its five-person crew in the north Atlantic.

The submersible, the Titan, had been in the area to explore the wreck of the Titanic when it lost contact on Sunday morning with a chartered research ship at the dive site. At the time, the 22-foot-long vessel was more than halfway into what should have been a two-and-a-half-hour dive.

The submersible is thought to be equipped with only a few days’ worth of oxygen, and as of 1pm Eastern time on Tuesday (10.30pm IST) there was probably about 40 hours of breathable air left, said Captain Jamie Frederick of the US Coast Guard.

He added that an area larger than Connecticut was being searched, but those efforts “have not yielded any results” and more ships and aircraft were heading to the site.

Even if the Titan can be located — in a remote patch of ocean where the seafloor lies more than 3km below the choppy surface — retrieving it will not be easy. That is partly because even the best divers cannot safely go more than a few hundred feet below the surface.

To recover objects off the seafloor, the US navy uses a remote-operated vehicle that can reach depths of 6,096 metres. But ships that carry such a vehicle normally move no faster than about 20kmph, and the Titanic wreck lies about 595km off the coast of Newfoundland, Canada.

US Coast Guard Rear Admiral John Mauger in television interviews on Tuesday said rescuers continued their efforts overnight and were expanding their search into deeper waters, telling NBC News that authorities were prioritising the area where the vessel was operating.

Those aboard the submersible, the highlight of a tourist expedition that costs $250,000 per person, included British billionaire Hamish Harding and Pakistani businessman Shahzada Dawood with his son Suleman.

US and Canadian ships and planes have been sweeping the area about 1,450km east of Cape Cod, some dropping sonar buoys that can monitor to a depth of 3,962 metres.

If the submersible experienced an emergency in mid-dive, the pilot would likely have released weights to float back to the surface, according to Alistair Greig, a professor of marine engineering at University College London. But absent any communication, locating a van-sized submersible in the vast Atlantic could prove challenging, he said.

If the Titan is on the ocean floor, a rescue effort would be difficult due to the extreme conditions more than 3km below the surface.