The journey to the ocean floor to reach the wreck of the Titanic is one that gets relentlessly colder and darker, says one of the handful of people who have ever visited the luxury liner's watery grave.
Tom Zaller, who runs the company behind "Titanic: The Exhibition," said touring the ship's resting place in a tiny submersible -- like the one that vanished Sunday in the North Atlantic -- was unforgettable, but frightening.
"As you get deeper and deeper, it gets darker and darker," he told AFP of his voyage, 23 years ago.
"When you when you first start off on the top, it's quite warm inside. But as you descend, it gets cold.
Zaller, whose exhibition opens in Los Angeles at the end of the month, said he was hoping desperately that the missing submersible could be found before its oxygen supplies run out -- estimated to be some time Thursday.
The 21-foot (6.5-meter) tourist craft lost communication with its mothership less than two hours into its trip.
The submersible, named Titan, was carrying British billionaire Hamish Harding and Pakistani tycoon Shahzada Dawood and his son Suleman, who also have British citizenship, on $250,000 tickets.
Also on board is the company's CEO, Stockton Rush, and a French submarine operator Paul-Henri Nargeolet, nicknamed "Mr Titanic" for his frequent dives at the site.
- Captain's bathtub -
More than two decades ago, Zaller travelled to the site, 400 miles (650 kilometers) from the nearest land as part of a research trip, aboard a Russian vessel with two submersibles.
"The sub is a two-meter wide pressured sphere," he said. "There's a pilot seat in the center, and then two benches on either side, with three portholes.
"On top there's an entry portal, and when you when you climb down inside the sub you then close it from the inside and there's another trap on the outside and once you're in, there's no going back. It's quite a commitment."
The sub is hoisted off the deck of the ship into blue waters which quickly turn dark as the vessel begins to sink.
For two and a half hours there is virtually nothing to see, with the sub conserving its power to use at the seafloor.
Then you reach the bottom, where the sub kicks up silt.
"When you look out the portal, it's a little cloudy. And then as you start to fly, when you get your buoyancy, you start to travel forward, you kind of come through this cloud.
"And then imagine coming through that cloud and being in this perfectly still environment on the bottom of the ocean, you know 3,800 meters below the surface and then you see a piece of debris, a giant piece of the Titanic.
"And then you see a cup or a teapot, and then at other moments you can see where the side of the ship is removed and you can see Captain Smith's bathtub full of water."
- 'Terrified' -
Zaller said he was nervous on his voyage to the ocean floor, despite the obvious professionalism and attention to detail by those running the trip.
"But still you're sending a very small vessel two and a half miles down, which is incredibly complicated and technical," he said.
"It's just this very seemingly unsophisticated sphere.
"I took a video camera and a video of myself. And I watched it later and I was absolutely terrified."
Zaller has known sub pilot Nargeolet for decades and was in touch with CEO Rush just before he embarked on this tour.
He says he is hoping against hope that everything will work out.
"I was in that sub for 12 hours with everything working fine," he said. "They've been there for almost four days. I just can't imagine.
"I hope and pray that they'll be okay and that they'll find them."
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