The missing Titanic tourists are understood to have signed a waiver that describes the submersible vessel they were travelling in as "experimental" and warns of possible death three times on the first page.
In a clip of him ahead of the expedition, a US journalist who joined a previous OceanGate expedition to see the wreckage said he had been made to sign a waiver acknowledging that the specially-designed submersible vehicle was "unregulated".
The disclaimer, read out by CBS correspondent David Pogue, read: "This experimental submersible vessel has not been approved or certified by any regulatory body, and could result in physical injury, disability, emotional trauma, or death."
A nervous-looking Pogue makes a face and says, "Where do I sign?" in the footage recorded when he went on the $250,000 (£195,000) trip to see the Titanic at the end of last year.
The latest expedition to see the shipwreck - located about 3,800 metres deep off the coast of Newfoundland, Canada -has now sparked a massive search and rescue operation in the North Atlantic Ocean.
Rescue teams are frantically searching for the submersible with time ticking after it went missing with a British billionaire among five people aboard.
Mike Reiss, who also took the Titanic submersible trip last year, told BBC Breakfast: "You sign a massive waiver that lists one way after another that you could die on the trip. They mention death three times on page one so it's never far from your mind.
"As I was getting onto the sub my thought was, this could be the end.
"So nobody who's in this situation was caught off guard. You all know what you are getting into.
"It is really exploration. It is not a vacation. It's not thrill seeking, it's not sky diving. These are explorers and travellers who want to see something."
The US Coast Guard said on Twitter that a boat on the surface - the Polar Prince - lost contact with the sub, called the Titan, about an hour and 45 minutes after it began diving toward the site of the Titanic's wreckage on Sunday morning.
US and Canadian ships and planes have swarmed the area about 900 miles (1,450 km) east of Cape Cod, some dropping sonar buoys that can monitor to a depth of 13,000 feet (3,962 metres), US Coast Guard Rear Admiral John Mauger said on Monday.
"It is a remote area and it is a challenge to conduct a search in that remote area," Mr Mauger said. "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."
Mr Mauger said officials have also been reaching out to commercial vessels for help.
The private company that operates the sub, OceanGate Expeditions, said in a statement on Monday that it was "mobilising all options" to rescue those on board.
An expert on the history of the Titanic said he was "very worried about the souls" onboard the missing submersible.
Tim Maltin, an author, historian and TV presenter said the tourists were in a difficult situation.
Mr Maltin said on BBC Breakfast: "If it's near the Titanic it would be easier to find, but the problem is of course you can't do a ship-to-ship transfer even, the pressure is absolutely intense.
"It's nearly two miles down, it's pitch black.
"So I also am very, very worried about the souls who are on board."
He added: "I think they're quite brave people who have been down there, but equally they know the risks but no one expects it to go wrong on your dive."
Additional reporting by agencies