The first British diver to see the Titanic wreck has paid tribute to his friend, the pilot of the stricken sub, saying he is an "extraordinary explorer" who knows the wreck "more than anyone else".
It comes as the search for the RMS Titanic Expedition Mission 5 submarine continues after last being seen on Sunday carrying passengers to the famous wreck.
The US Coast Guard is currently searching an area "about the size of Connecticut" for the missing submarine, which has five people inside.
Dik Barton made 22 dives to the wreck of the liner to recover artefacts when he was head of operations with RMS Titanic Inc, a US firm which salvaged the site.
He worked with French submersible pilot Paul-Henry Nargeolet on the dives, which would take two-and-half hours to descend to the wreck and half an hour less to surface again.
Mr Barton, a businessman and guest lecturer at Sunderland University, said the group on this current ill-fated trip had "good leadership".
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"He is the kind of man who will keep them calm, but it's not easy in a tube at the bottom of the ocean," he said. It's tragic.
"He is an extraordinary explorer and an incredible individual and he knows the wreck better than anybody I know."
"It's an incredibly hostile environment at the depths we are talking about.
"The pressure down there is 2,500lbs per square inch, that's the equivalent of two adult elephants on your thumb nail.
"If something goes wrong, it goes wrong very quickly."
He believed the submarine had lost either power on its descent or its structural integrity at great depth.
"It's completely pitch black and then you get to the sea bed, put on your lights and there you are with the Titanic before you, if you land in the right place.
"It's not easy, the wreck site is two miles across."
The Titanic retained its grip on the public consciousness, Mr Barton said.
"There's much more glamorous and more incredible maritime disasters and wrecks, but this one has been romanticised over the years," he said.
"It is a place of fable that people want to go and see, and they are prepared to pay a lot of money to do so."
He has previously said to picture the scene, people should imagine the "best hotel in the world 600 miles off the coastline, crack it in half like and egg and spread the contents on the seabed".
