US and Canadian ships and planes are searching for a submersible craft missing off the coast of southeastern Canada in the Atlantic while taking tourists to explore the wreckage of the Titanic.
The United States Coast Guard said on Monday there was one pilot and four passengers on board and that the vessel had the capacity to be submerged for 96 hours, but it was unclear whether it was still underwater or had surfaced and was unable to communicate.
Ships and planes have swarmed the area about 1450km east of Cape Cod, some dropping sonar buoys that can monitor to a depth of 13,000 feet (3962 metres), US Coast Guard Rear Admiral John Mauger told reporters.
"It is a remote area and it is a challenge to conduct a search in that remote area. We are deploying all available assets to make sure that we can locate the craft and rescue the people on board. Going into this evening we will continue to fly aircraft and move additional vessels."
Mauger said officials have also been reaching out to commercial vessels for help.
British billionaire Hamish Harding is among the passengers, according to a social media post from a relative.
The US Coast Guard said earlier on Twitter that a boat on the surface - the Polar Prince - lost contact with submersible, called the Titan, about one hour and 45 minutes after it began diving toward the site of the Titanic's wreckage on Sunday morning (local time).
OceanGate said: "We are deeply thankful for the extensive assistance we have received from several government agencies and deep sea companies in our efforts to re-establish contact with the submersible."
Harding's stepson wrote on Facebook that Harding had "gone missing on submarine" and asked for "thoughts and prayers." The stepson subsequently removed the post, citing respect for the family's privacy.
Harding had posted on Facebook a day earlier that he would be aboard the sub. There have been no posts from him since.
The expedition headed out to sea on Friday, and the first dive was set for Sunday morning, according to his post.
The expedition, which costs $US250,000 ($NZ403,000) per person, starts in St John's, Newfoundland, before heading out approximately 640km into the Atlantic to the wreckage site, according to OceanGate's website.
In order to visit the wreck, passengers climb inside Titan, the five-person submersible, which takes about two hours to descend approximately 12,500 feet (3800 metres) to the Titanic.
The expedition company said that the craft has life support for its standard load of five people for 96 hours.
The British passenger ship famously sank in 1912 on its maiden voyage after striking an iceberg, killing more than 1500 people.
The story has been immortalised in nonfiction and fiction books as well as the 1997 blockbuster movie Titanic.
